Automated Checking of Flexible Mathematical Reasoning in the Case of

Systems of (In)Equations and the Absolute Value Operator

Antti Valmari

University of Jyv

askyl

a, Faculty of Information Technology, PO Box 35, FI-40014 University of Jyv

askyl

a, Finland

Keywords:

High School Mathematics, Elementary University Mathematics, Automatic Feedback, eLearning Software,

Non-trivial Reasoning Patterns, Higher Order Thinking Skills.

Abstract:

We present an approach and a tool for automatically providing feedback on solutions that involve complicated

reasoning patterns. Currently the tool supports linear systems of equations and inequations that may also

contain the absolute value operator and a restricted form of rational functions. This sufﬁces for designing

problems that are laborious to solve with standard mechanical procedures, but much easier using short-cuts that

students may ﬁnd by creative thinking. Earlier research has found that struggling with important mathematics

promotes conceptual development. Our goal is to encourage students to such struggling. A crucial feature is

to give them great freedom to choose the paths via which they solve problems, and at any time ask the tool

to check the work done so far, no matter what path was chosen. This was implemented by adopting standard

notation from mathematical logic, and developing some new logical notation. The tool has been used in a

course on elementary university-level mathematics. It has worked reliably, but there is not yet any statistics on

the pedagogical merits. The tool is expected to also support quadratic (in)equations in the near future.

1 INTRODUCTION

The book “Adding It Up: Helping Children Learn

Mathematics” by the National Research Council char-

acterizes mathematical proﬁciency as consisting of

ﬁve strands: conceptual understanding, procedural

ﬂuency, strategic competence, adaptive reasoning and

productive disposition (Kilpatrick et al., 2001, p. 5).

The extent to which the development of each of these

can be supported by modern computer tools varies.

Automatic answer checkers are widely used for de-

veloping procedural ﬂuency. Conceptual understand-

ing beneﬁts from playing with tools that illustrate and

facilitate experimenting with mathematical concepts.

The present study addresses the use of our com-

puter tool to develop adaptive reasoning, deﬁned

by (Kilpatrick et al., 2001) as capacity for logi-

cal thought, reﬂection, explanation, and justiﬁcation.

(Anthony and Walshaw, 2009) observed that “Ques-

tions that have a variety of solutions or can be solved

in more than one way have the potential to provide

valuable insight into student thinking and reasoning.”

We believe that such questions also have the potential

to develop the students’ reasoning skills. To illustrate

https://orcid.org/0000-0002-5022-1624

our idea, consider the following problem. It is from

the famous book “How to Solve It” (P

olya, 1945).

Solve the following system of equations.

x +7y + 3v + 5u = 16

8x + 4y + 6v + 2u = −16

2x + 6y + 4v + 8u = 16

5x + 3y + 7v + u = −16











(1)

Of course, this system can be solved by following

blindly a mechanical procedure. For instance, one

could eliminate variables one at a time by solving a

variable from one equation and assigning the solution

to the remaining equations. Doing this for x and the

ﬁrst equation produces the following:

x = 16 − 7y − 3v − 5u

8(16 − 7y − 3v − 5u) + 4y + 6v + 2u = −16

2(16 − 7y − 3v − 5u) + 6y + 4v + 8u = 16

5(16 − 7y − 3v − 5u) + 3y + 7v + u = −16

Carrying the solution through with this approach

would consist of lots of tedious, error-prone, unmo-

tivating work.

olya wanted the student not to take that ap-

proach. His book is about developing general mathe-

matical problem solving skills. His intention was that

the student should ﬁnd clever short-cuts via which the

system could be solved with much less work.

324

Valmari, A.

Automated Checking of Flexible Mathematical Reasoning in the Case of Systems of (In)Equations and the Absolute Value Operator.

DOI: 10.5220/0010493103240331

In Proceedings of the 13th International Conference on Computer Supported Education (CSEDU 2021) - Volume 2, pages 324-331

ISBN: 978-989-758-502-9

For instance, one could start by adding the second

original equation to the third, resulting in

10x + 10y + 10v + 10u = 0 (2)

Adding the ﬁrst and last original equation yields

6x + 10y + 10v + 6u = 0 (3)

By subtracting (3) from (2) we get

4x + 4u = 0

from which obviously u = −x. Assigning −x in the

place of u in (2) or (3) yields 10y + 10v = 0, that is,

v = −y. These facilitate immediate simpliﬁcation of

the ﬁrst two original equations to

−4x + 4y = 16

6x − 2y = −16

Adding them gives 2x + 2y = 0, implying y = −x.

This together with 6x − 2y = −16 yields 8x = −16.

We conclude x = −2, y = 2, v = −2 and u = 2.

The idea is thus to give students problems that can

be solved with little work if they use some creativity,

but are laborious or impossible to solve by blindly fol-

lowing mechanical procedures. The present paper is

about how to support this idea with automated feed-

back or automated checking of the student’s solution.

Of course, traditional automatic checking of ﬁnal

answers can be employed to check that, for instance,

in the above example, the student ended up with the

correct values x = −2, y = 2, v = −2 and u = 2. How-

ever, as such, it does not sufﬁce. Assume that the stu-

dent made a sign error and, instead of (3), got

6x + 10y + 10v + 6u = 32 (4)

Continuing like above results in x = −8, y = 0, v = 8

and u = 0. Traditional automatic checking can only

tell that it is incorrect and reveal the correct answer.

This does not give the student much indication of

where the error took place. Furthermore, the student

has spent a lot of effort in progressing from (4) to

the incorrect ﬁnal answer. This work is now wasted,

which is frustrating.

We have developed a tool with which the stu-

dent can check each step immediately after writing

it (Kaarakka et al., 2019; Valmari and Rantala, 2019).

If the student asks the tool to check (4), the tool tells

that it is incorrect. So the student becomes immedi-

ately aware of the problem and can focus on ﬁxing

it, instead of ﬁrst doing a lot of futile work in devel-

oping an incorrect ﬁnal answer. We are aware of no

other tool that supports such step-by-step checking of

both arithmetic and logical reasoning.

The students have full control. They choose the

paths via which to solve the problems: which variable

to solve ﬁrst and from which equation; or not to start

like that at all and instead add equations to each other

like was done above; or do something else. They

choose what intermediate steps to write and what to

perform only in their heads. They choose when to ask

the tool to check what they have written.

A student that is ﬂuent with a problem may pro-

ceed in big steps. If the tool accepts the step, great;

and if not, then the student may add sub-steps to make

the tool tell the location of the error more precisely. A

newcomer may proceed in very small steps, pressing

the check button after each step.

The tool is not restricted to systems of linear equa-

tions. Currently it also supports ≤, <, ≥, >, 6=, ab-

solute values, a restricted form of rational functions,

and a range of logical symbols including quantiﬁers.

Univariate quadratic rational inequations with abso-

lute values are being implemented. The tool works by

using advanced algorithms from mathematical logic

to check whether each step written by the student is

logically correct or not. In many cases this means that

the tool need not know in advance anything about the

problem that the student is solving. As a consequence,

a teacher may give students new problems without

feeding in any model solution or similar. This is the

case, for instance, with the problem (1) by P

olya.

Alternatively, the teacher may give a mathemati-

cal formulation of a problem to the tool, and present

the problem to the students in natural language. Then

it is a part of the task of the students to translate the

problem from natural language to a mathematical rep-

resentation. The formulation need not be precisely

the same as the teacher’s; it sufﬁces that it is logically

equivalent.

Of course, for this to work, there must be a no-

tation in which the students express their reasonings.

The tool uses symbols from mathematical logic and

a small number of additional symbols. The latter are

needed to express non-trivial patterns of reasoning in

a practical way.

The features described in this paper have been im-

plemented only recently. They have been technically

tested and many of them have been used in a real-

life course. There is, however, not yet any systematic

statistics from the pedagogical point of view.

In the next section we introduce the basic concepts

of our approach, only using a straightforward reason-

ing pattern. Section 3 discusses various kinds of feed-

back that our tool gives. The topic of Section 4 is

how to express complicated claims such as systems

of equations. Reasoning patterns that are not straight-

forward are illustrated in Section 5. The section con-

tains the solution to (1) developed above, in a form

accepted by our tool. Beyond the most elementary

Automated Checking of Flexible Mathematical Reasoning in the Case of Systems of (In)Equations and the Absolute Value Operator

325

|2x-1| = 5

<=> 2x-1 = -5 \/ 2x-1 = 5

<=> x = -2 \/ x = 3

Figure 1: An example input by the student.

Figure 2: The output by the tool for the input in Figure 1.

notions, the notation system for logic has not become

standardized. In Section 6 we compare our notation

to those found in textbooks. Our concluding remarks

are in Section 7.

2 BASIC CONCEPTS

The key technical concepts in this paper are formulas

and reasoning steps. Figure 1 shows a reasoning that

consists of three formulas that have been organized

into two reasoning steps. It has been written in the in-

put language of our tool. Figure 2 shows the response

by our tool to the input in Figure 1. It consists of a

copy of the input translated to standard mathematical

notation, followed by the verdict that the reasoning

was correct.

The ﬁrst formula in Figure 1 (or Figure 2) ex-

presses the equation |2x − 1| = 5. We call it the origi-

nal problem, because it is what the student is expected

to solve. The roots of an equation are the values of the

unknown that make the equation hold. For instance,

the roots of |2x − 1| = 5 are −2 and 3.

The last formula of a reasoning is called the ﬁnal

answer. In this case it is x = −2 ∨ x = 3. It says that

x is −2 or x is 3 (∨ denotes the logical or). Because it

lists the roots as the possible values of x, it is correct.

The second formula 2x−1 = −5 ∨ 2x−1 = 5 is an

intermediate result. A reasoning that solves a difﬁcult

problem may contain many intermediate results.

In general, a formula expresses a claim about the

values of the variables in question. As was illustrated

above, the original problem is represented as a for-

mula, the ﬁnal answer is represented as a formula, and

each intermediate result is represented as a formula.

In this section, we say that a formula is correct if

and only if it is true exactly when the original prob-

lem is true. (This becomes more complicated in Sec-

tion 5.) For instance, every formula in Figure 2 is

true when x is −2 or 3, and everyone is false when x

has any other value, so they are all correct. This no-

tion is different from “tautology” and “valid formula”

in mathematical logic (they mean, roughly speaking,

that the formula is always true). The original problem

is correct by its nature, but intermediate results and

the ﬁnal answer may be incorrect. It is the task of our

tool to check whether they are correct.

A reasoning step consists of two formulas and a

reasoning operator that relates them. Until Section 5,

we only use one reasoning operator ⇔, meaning that

the formula on its left hand side is true exactly when

the formula on its right hand side is true. If this holds,

we call the reasoning step correct, and otherwise in-

correct. For instance, if x = −2, then both |2x−1| = 5

and x = −2 ∨ x = 3 are true; if x = 3, then both

are true; and if x has any other value, then both are

false (and thus not true). Therefore, |2x − 1| = 5 ⇔

x = −2 ∨ x = 3 is a correct reasoning step.

A counter-example to an incorrect ⇔-step is val-

ues of the unknowns such that one side of the step is

true and the opposite side is not true. Every incorrect

and no correct step has a counter-example. For in-

stance, |2x − 1| = 5 ⇔ x = 3 has the counter-example

x = −2, because it makes |2x − 1| = 5 true (indeed,

|2 · (−2) − 1| = 5) but x = 3 false (because −2 6= 3).

Successive reasoning steps may share a formula.

Figure 1 (and 2) shows two reasoning steps that share

the formula 2x − 1 = −5 ∨ 2x − 1 = 5.

If every reasoning step in a reasoning of the form

“original problem ⇔ formula ⇔ . . . ⇔ formula” is

correct (in the sense of correctness of ⇔-steps just

introduced), then every formula in the reasoning is

correct (in the sense of correctness of formulas intro-

duced earlier on). This property is known as transi-

tivity.

More reasoning operators will be introduced in

Section 5. Each of them has its own notion of cor-

rectness.

3 FEEDBACK BY THE TOOL

Figure 3 shows an example of the feedback by the

tool to an incorrect input. It says that if a = −3, then

|a − 3| = 2a is false (F) but a −3 = −2a ∨ a −3 = 2a

is true (T). Indeed, if a = −3, then |a −3| = | − 6| = 6

but 2a = −6, so |a − 3| = 2a is false. Furthermore,

a − 3 = 2a is true, making a − 3 = −2a ∨ a − 3 = 2a

true. That is, the tool gave a counter-example to the

incorrect reasoning step. We see that a − 3 = 2a may

hold such that 2a is negative, and then |a − 3| = 2a

does not hold because |anything| is non-negative. We

will discuss a correct solution in Section 4.

According to (Hiebert and Grouws, 2007, p. 383,

CSEDU 2021 - 13th International Conference on Computer Supported Education

326

Figure 3: The output by the tool for an incorrect input.

387), struggling with important mathematics is a key

feature that promotes conceptual development. They

emphasize that struggling does not mean “needless

frustration” in front of an impossible task. It means

that “students expend effort to make sense of math-

ematics, to ﬁgure something out that is not imme-

diately apparent.” We believe that in many cases,

a counter-example may be a good starting point for

such struggling. For instance, the counter-example

in Figure 3 may lead the student to realize that when

|. . . | is replaced by . . . or −(. . .), it may be necessary

to ensure that the substitute is non-negative.

Sometimes every existing counter-example is a

root of the original problem. This can be intentionally

caused by giving F as an intermediate result. Then

the counter-example reveals the correct ﬁnal answer

or a part of it. This is not a big issue, if the inten-

tion of the student is not to cheat but to learn. Fur-

thermore, it is possible to switch off the printing of

counter-examples.

It may also happen that incorrect thinking leads to

a correct result by accident. Consider again |a − 3| =

2a. A student may plan to write a−3 = −2a ∨ a−3 =

2a, write a − 3 = −2a, and be interrupted by a phone

call. After the call (s)he no longer remembers what

(s)he was doing. There is a − 3 = −2a on the screen,

so (s)he may continue by solving it. Altogether (s)he

wrote |a − 3| = 2a ⇔ a − 3 = −2a ⇔ a = 1.

The tool accepts this as correct, because the same

solution could have arisen from correct thinking: An

experienced student sees quickly that the root of a −

3 = 2a is a = −3. However, it cannot be a root of

the original equation, because it makes 2a negative.

Therefore, there is no need to write a branch for the

case where a − 3 = 2a.

Of course, the tool rejects |a − 3| = 2a ⇔ a − 3 =

2a. If the student solves many and diverse enough

exercises, the odds are that most misunderstandings

are caught sooner or later, although not necessarily at

the ﬁrst time when the student makes the error.

Figure 4 shows the reply by the tool when the stu-

dent has not developed the answer to an explicit form.

The formula x = −2 ∨ 2x = 6 is correct, that is, it is

true precisely when the original formula |2x − 1| = 5

is true. However, the root x = 3 is not directly shown

Figure 4: An incompletely solved example.

in the formula, but only indirectly via 2x = 6. Be-

cause the answer is mathematically correct, and the

problem is only that the student has not simpliﬁed it

far enough, the tool shows the remark in magenta in-

stead of red.

In the most typical case, an explicit form is x = c

∨ ··· ∨ x = c

, where x is the unknown and the c

are constants. Also the opposite direction c

= x is

allowed for the reason discussed soon. If the equation

has no roots (e.g., x = x + 1), then the explicit form

is F, and if every real number is a root (e.g., x + 1 =

x + 1), then it is T.

The set of roots may also contain intervals. This

is obvious in the case of inequations (e.g., Figure 5),

but may also occur with equations using the absolute

value operator (e.g., |x| = x ⇔ x ≥ 0). Therefore, also

x ≥ c

, c

< x ≤ c

, etc., are treated as being in the

explicit form. Alternatively, c

< x ≤ c

can be written

using the logical and as c

< x ∧ x ≤ c

Figure 5 was obtained with the input

(2x+1)/(5x-2) >= 0 <=> x <= -1/2 \/ x >= 2/5

If the latter >= in the input is replaced by >, the tool

accepts the input as correct.

Figure 5 also illustrates that our tool uses a third

truth value “undeﬁned” (U). This is a long story, but

let us quickly mention some most important aspects.

When x =

2x+1

5x−2

is undeﬁned, because the divisor

5x − 2 is 0. (In)equalities whose one or both sides is

undeﬁned must not be treated as true, because other-

wise the corresponding value of the unknown would

be a root. Therefore, with only F and T available,

both

2x+1

5x−2

≥ 0 and

2x+1

5x−2

< 0 should be F when x =

On the other hand, we would expect ¬(

2x+1

5x−2

≥ 0) to

2x+1

5x−2

< 0, and ¬F is T (¬ is the logical not). So

2x+1

5x−2

< 0 should be both F and T when x =

, which

is impossible.

Figure 5: Intervals of roots, and division by zero.

Automated Checking of Flexible Mathematical Reasoning in the Case of Systems of (In)Equations and the Absolute Value Operator

327

Figure 6: An unnecessarily complicated ﬁnal answer.

This obstacle is removed by treating

≥ 0 neither

as true nor as false, but as undeﬁned. The negation

of undeﬁned is undeﬁned as well. The law ¬(x ≥ y)

⇔ x < y remains valid even if x or y is undeﬁned.

“Formula1 ⇔ formula2” still means that formula1 is

true precisely when formula2 is true. This works even

if formula1 yields U and formula2 yields F, because

neither U nor F is T. Because ⇔ resides not in but

between formulas, it cannot be within the scope of ¬.

Currently the tool makes some attempts to check

that the ﬁnal answer is not more complicated than

necessary. An example of this is shown in Figure 6.

However, the tool could do much more towards this

end than it currently does. This is work in progress.

If the teacher is willing to touch the tool, (s)he can

set an upper bound to the length of the ﬁnal answer.

This has proven a very efﬁcient method of catching

too complicated ﬁnal answers.

4 USE OF LOGICAL AND

The equations in a system of equations are intended

to hold simultaneously. This can be obtained by com-

bining them with the logical and ∧. Similarly, the sub-

formulas that tell the simultaneous values of different

unknowns are combined with ∧. Figure 7 shows an

example of both of these. The ﬁgure also illustrates

that if a system with two unknowns has more than

one root, the alternative roots are separated with ∨.

Let us return to the example in Figure 3. By def-

inition, |x| = x if x is positive or zero, and |x| is −x

if x is negative. Therefore, the (or at least a) correct

method to solve |a −3| = 2a is to split it to two cases.

In one case a − 3 is positive or zero, and a − 3 is used

in the place of |a −3|. It can be written as a −3 ≥ 0 ∧

a − 3 = 2a. Solving a − 3 = 2a yields a = −3. How-

ever, if a = −3, then a −3 ≥ 0 does not hold, so −3 is

not a root in the end. In the other case a − 3 is nega-

tive, and −(a − 3) is used in the place of |a − 3|. That

is, a −3 < 0 ∧ −(a−3) = 2a. We have −(a−3) = 2a

⇔ a = 1. If a = 1 then a− 3 < 0 holds, so 1 indeed is

Figure 7: Two examples of the use of ∧.

Figure 8: A clumsy solution to |a − 3| = 2a.

a root. In conclusion, |a − 3| = 2a ⇔ a = 1.

This reasoning can be expressed with ∧, ∨ and

⇔, but only in a clumsy fashion. Figure 8 shows it.

On the second, third and fourth line, the two cases

are combined with ∨. The two equations a − 3 = 2a

and −(a − 3) = 2a are solved in lockstep, although

it would be more convenient to solve them one at a

time. The problem is that with no other reasoning

operator available than ⇔, each intermediate result

must reﬂect the original problem in full; that is, it is

not possible to say that for a while, we focus on one

aspect of the original problem and ignore the rest. In

the next section, we introduce notation that removes

this restriction and facilitates handy representation of

many reasoning patterns.

Our tool also knows the symbols ¬ (not), → (if

. . . then) and ↔ (if and only if). For instance, the

tool accepts the reasoning step ¬(x < 0) ⇔ x ≥ 0.

The second line in Figure 8 can also be represented as

(a −3 ≥ 0 → a −3 = 2a) ∧ (a − 3 < 0 → −(a − 3) =

2a), and some people ﬁnd it more natural.

5 MORE REASONING PATTERNS

In the previous section we only used a straightfor-

ward type of reasoning, where the formulas are sepa-

rated with ⇔, and each formula must be true precisely

when the original problem is true. In this section we

discuss more general patterns of reasoning. They are

important for giving the student the freedom to choose

the path via which to solve the problem.

The key concept is logical implication, expressed

with ⇒ and in reverse direction with ⇐. The rigorous

deﬁnition is that “formula1 ⇒ formula2” is a correct

reasoning step if and only if all those value combina-

tions of the unknowns that make formula1 true, also

make formula2 true. For instance, Figure 9 tells that

|x| + y = 6 ∧ 3x + 5y = 22 ⇒ x = 4 ∧ y = 2 is incor-

rect because of the counter-example x = −1 ∧ y = 5,

but the opposite direction is correct.

There is evidence that the asymmetry of ⇒ is very

difﬁcult for many students. For instance, the ACM &

IEEE curriculum recommendation speciﬁes the cor-

responding topic with exceptional detail: “Notions

CSEDU 2021 - 13th International Conference on Computer Supported Education

328

Figure 9: An incorrect implication whose converse is cor-

rect.

of implication, equivalence, converse, inverse, con-

trapositive, negation, and contradiction” (ACM and

IEEE., 2013, p. 78). A hopefully helpful way to think

of it is that if and only if “formula1 ⇒ formula2”,

then formula2 contains the same or strictly less in-

formation about the values of the unknowns than for-

mula1. For instance, 3 < x ≤ 5 contains less informa-

tion about the value of x than 2 ≤ x ≤ 4, because the

latter speciﬁes the value of x more precisely.

Consider again (1), with ∧ added to the end of

its ﬁrst three lines. Intuitively, picking two equations

from among the four amounts to throwing some in-

formation away and keeping the rest. Therefore, we

should have “all four in (1) ⇒ the 2nd and 3rd in (1)”.

The ﬁrst two lines of Figure 10 show that our tool is

of the same opinion. (The ﬁgure also illustrates that

the student may write comments in a solution, and the

tool copies them to the output in brown colour.)

As a matter of fact, “formula1 ⇒ formula2” ap-

plies to any reasoning that does not use other situ-

ation-speciﬁc information than what is in formula1.

(General laws such as x + 0 = x are not situation-spe-

ciﬁc information.) Adding two equations only uses

information in those equations (in addition to general

laws), so also the third line in Figure 10 is correct.

The fourth line uses the keyword original to

handily refer to the original problem. It derives the

sum of the ﬁrst and fourth original equation without

showing any intermediate results. Because the formu-

las on the third and fourth line follow from the orig-

inal problem, also the result of subtracting one from

the other does. This is shown on the ﬁfth line, fol-

lowed by the solving of u in terms of x.

The solution described in Section 1 could be car-

ried all the way through only using original, ⇒

and ⇔, but we chose to use also other features, to il-

lustrate them. An arbitrary reasoning (that is within

the capacity of the tool) may be enclosed between

subproof and subend. The ﬁrst such instance in Fig-

ure 10 takes the results from the third and ﬁfth line,

and reasons v = −y from them.

The second instance starts by lifting u = −x ∧ v =

−y to the status of a general law that is valid through-

out the instance. This makes it possible to compare

x + 7y + 3v + 5u = 16 ∧ 8x + 4y + 6v + 2u = −16 to

−4x + 4y = 16 ∧ 6x − 2y = −16 with ⇔ instead of

⇒, although only the former uses u and v. As a con-

sequence, the tool checks fully the correctness of the

elimination of u and v, while ⇒ would have allowed,

for instance, −4x + 4y = 16 as such (that is, without

6x − 2y = −16) on the right hand side.

This is because ⇒ allows throwing any informa-

tion away. It is used to intentionally throw some infor-

mation away. However, the logic and the tool do not

know which pieces of information were thrown away

intentionally and which accidentally, so ⇒ must al-

low throwing any information away.

This issue does not make the tool accept incorrect

ﬁnal answers. If the reasoning as a whole fails to es-

tablish the ⇔-relation between the original problem

and the ﬁnal answer (for instance, only establishes the

⇒-relation), then the tool says in magenta that “Im-

plication was used without returning to the original”.

However, it can cause the error message occur much

later than where the actual error was, potentially con-

fusing students.

The second subproof in Figure 10 continues in an

already seen fashion to the value of y in terms of x, and

to the numeric value of x. Here “original” refers to

the ﬁrst formula of the subproof after the assumption.

Having derived u = −x, v = −y, y = −x and x = −2,

the ﬁnal answer is easy to write.

We have now presented, in a form that our tool

was able to check, the reasoning in Section 1 that

solved P

olya’s problem via short-cuts. The reasoning

could be made shorter still by replacing the subproofs

with original ⇒ -structures, but we chose to use this

opportunity to illustrate subproofs and assumptions.

Let us discuss some further examples that illus-

trate other aspects of solving equations. Figure 11

shows an example of using assumptions to express the

conditions of the cases of an absolute value operation.

In the ﬁrst branch, thanks to the assumption x ≥ 0,

we have |x| = x and thus |x| + y = 6 ⇔ x + y = 6 ⇔

x = 6 − y. Similarly in the second branch |x| + y = 6

⇔ −x + y = 6 ⇔ x = y − 6. The advantage of this so-

lution is that it avoids ⇒ altogether. It thus avoids the

risk of postponed error messages mentioned above.

A solution by cases can be written without sub-

proofs by starting the ﬁrst case with ⇐ and the re-

maining cases with original ⇐. This works because

each case contains at least the same information as

the original problem: all (in)equations in the original

problem apply, and also the (in)equation that speci-

ﬁes the case applies. Analogously to ⇒, ⇐ runs the

risk of accidentally putting too much information to a

Automated Checking of Flexible Mathematical Reasoning in the Case of Systems of (In)Equations and the Absolute Value Operator

329

Figure 10: Solving P

olya’s problem (1).

case, causing an error that the tool detects later than

where it actually occurred.

With the next version of the tool,

−9x+18

+2x−15

= 0

can be solved by solving x

− 9x + 18 = 0, and then

rejecting those roots that make x

+2x −15 zero. The

roots are 3 and 6, of which 3 must be rejected. The

advantage is that x

+2x − 15 = 0 need not be solved.

This solution can be presented with ⇒ or a subproof.

By assuming x

+ 2x − 15 6= 0 in the latter, ⇒ can be

avoided. (All this already works on a prototype.)

6 ON ALTERNATIVE NOTATIONS

Although our use of ⇔, ⇒ and ⇐ agrees with fairly

common informal practice, the reader is warned that

Figure 11: Exploiting assume in a solution by cases.

textbooks on mathematical logic tend to disagree with

both our use and each other. We make a distinction

between → that resides in a formula and produces a

truth value; and ⇒ that resides between two formu-

las and expresses a correct or incorrect reasoning step

(and similarly with ↔ and ⇔). Most if not all authors

introduce the former notion. Many denote it with →,

but many others use ⇒ instead. Most authors avoid

the latter notion altogether, use ad-hoc notation, or

confuse it with the former notion.

For instance, (Hammack, 2018) uses ⇒, ⇔ and =

in the roles of our →, ↔ and ⇔ (e.g., p. 51: P ⇔ Q

= (P ∧ Q) ∨ (∼ P∧ ∼ Q)). It apparently lacks our ⇒.

(Hein, 1995) uses → similarly to us, ≡ similarly to

our ⇔, and lacks our ↔ and ⇒. (Huth and Ryan,

2004) use → similarly to us, do not use ↔, and use `

and a` somewhat similarly to our ⇒ and ⇔.

Our ⇒ and ⇔ are relational: P ⇒ Q ⇒ R is cor-

rect if and only if P ⇒ Q is correct and Q ⇒ R is cor-

rect. This is crucial for handy writing of reasonings,

which in turn is central to our goal of supporting the

development of adaptive reasoning.

Unfortunately, the existence of different conven-

tions can lead to dramatically different meanings

for the very same expression. For instance, (Stan-

ford University., 2021, Section 2.2) uses a right-

associative ⇒ in the role of our →. Under that con-

vention, P ⇒ Q ⇒ R means the same as P ⇒ (Q ⇒ R),

that is, Q ⇒ R is evaluated ﬁrst, and its result is then

used as the X in the evaluation of P ⇒ X. Consider

CSEDU 2021 - 13th International Conference on Computer Supported Education

330

x = 2 ⇒ x = 1 ⇒ x = 0. When x 6= 2, it is T because

it is then of the form F ⇒ something. When x = 2, it

is T because x = 1 ⇒ x = 0 is then of the form F ⇒ F

which is T, so the expression as a whole is of the form

T ⇒ T, and thus T. That is, although in our conven-

tion x = 2 ⇒ x = 1 ⇒ x = 0 is just plain wrong, under

the Stanford convention it is T for every value of x!

In our convention, P ⇒ (Q ⇒ R) is a syntax error.

The notations original, subproof, subend and

assume are by us, but of course the ideas are next to

trivial. Together with assume, original ⇒ is essen-

tially the same as Γ |= in hard-core formal logic.

The undeﬁned truth value U is almost never men-

tioned in textbooks. Its behaviour in the case of ¬,

∧, ∨, → and ↔ is fairly unproblematic; the tool fol-

lows (Kleene, 1952). Beyond that, in particular re-

garding ⇒ and ⇔, the issue becomes complicated

indeed. A number of different approaches were dis-

cussed by (Schieder and Broy, 1999). The tool uses

our own approach (Valmari and Hella, 2017).

7 CONCLUDING REMARKS

We illustrated how our tool can automatically check

various patterns of reasoning in a restricted domain

of problems, and give feedback that the student can

use as a starting point for improving the solution. Al-

though the domain is restricted, it can meaningfully

accommodate many non-straightforward patterns of

reasoning. The example by (P

olya, 1945) is certainly

challenging enough to bring forward the beneﬁts of

creative thinking over blindly following mechanical

procedures. It was successfully dealt with by the tool.

It is worth emphasizing that our tool facilitates

verbal problems that the students have to translate

to (in)equations themselves. The teacher gives the

(in)equation (or anything logically equivalent, such as

the roots) to the tool as the original problem and asks

the tool to keep it hidden from the students.

An earlier feature of our tool has been found to

statistically signiﬁcantly improve performance in an

examination (Kaarakka et al., 2019). The implemen-

tation of the features discussed in this paper started

in spring 2020. Therefore, there is not yet much ped-

agogical experience. In a course, students had little

problems with the tool itself, but had big problems

with mathematics that they were supposed to already

master: absolute values, pairs of equations, and so on.

They also had big problems in modelling verbally ex-

pressed problems mathematically. Their earlier stud-

ies had not developed these skills to the promised

level.

Until now it has not been possible to make stu-

dents solve this kind of problems in great numbers,

because of the lack of teachers to check the answers

and provide feedback. Now we have a tool for this.

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