2.6: Conjectures and Counterexamples

Educated guesses and examples that disprove them.

A conjecture is an “educated guess” that is based on examples in a pattern. A counterexample is an example that disproves a conjecture.

Suppose you were given a mathematical pattern like \(h = \dfrac{−16}{t^2}\). What if you wanted to make an educated guess, or conjecture, about \(h\)?

Use the following information for Examples 1 and 2:

A car salesman sold 5 used cars to five different couples. He noticed that each couple was under 30 years old. The following day, he sold a new, luxury car to a couple in their 60’s. The salesman determined that only younger couples by used cars.

Example \(\PageIndex{1}\)

Is the salesman’s conjecture logical? Why or why not?

Solution

It is logical based on his experiences, but is not true.

Example \(\PageIndex{2}\)

Can you think of a counterexample?

Solution

A counterexample would be a couple that is 30 years old or older buying a used car.

Example \(\PageIndex{3}\)

Here’s an algebraic equation and a table of values for \(n\) and \(t\).

\(t=(n−1)(n−2)(n−3)\)

\(n\)	\((n−1)(n−2)(n−3)\)	\(t\)
1	\((0)(−1)(−2)\)	0
2	\((1)(0)(−1)\)	0
3	\((2)(1)(0)\)	0

Solution

After looking at the table, Pablo makes this conjecture:

The value of \((n−1)(n−2)(n−3)\) is 0 for any number n.

Is this a true conjecture?

This is not a valid conjecture. If Pablo were to continue the table to n=4, he would have see that \((n−1)(n−2)(n−3)=(4−1)(4−2)(4−3)=(3)(2)(1)=6\)

In this example \(n=4\) is the counterexample.

Example \(\PageIndex{4}\)

Arthur is making figures for an art project. He drew polygons and some of their diagonals.

Figure \(\PageIndex{1}\)

From these examples, Arthur made this conjecture:

If a convex polygon has \(n\) sides, then there are \(n−2\) triangles formed when diagonals are drawn from any vertex of the polygon.

Is Arthur’s conjecture correct? Or, can you find a counterexample?

Solution

The conjecture appears to be correct. If Arthur draws other polygons, in every case he will be able to draw \(n−2\) triangles if the polygon has n sides.

Notice that we have not proved Arthur’s conjecture, but only found several examples that hold true. So, at this point, we say that the conjecture is true.

Example \(\PageIndex{5}\)

Give a counterexample to this statement: Every prime number is an odd number.

Solution

The only counterexample is the number 2: an even number (not odd) that is prime.

Review

Give a counterexample for each of the following statements.

1. If \(n\) is a whole number, then \(n^2 >n\).
2. All numbers that end in 1 are prime numbers.
3. All positive fractions are between 0 and 1.
4. Any three points that are coplanar are also collinear.
5. All girls like ice cream.
6. All high school students are in choir.
7. For any angle there exists a complementary angle.
8. All teenagers can drive.
9. If \(n\) is an integer, then \(n>0\).
10. All equations have integer solutions.

Review (Answers)

To see the Review answers, open this PDF file and look for section 2.5.

Resources

Vocabulary

Term	Definition
conjecture	A conjecture is an educated guess that is based on examples in a pattern.
counterexample	A counterexample is an example that disproves a conjecture.
fraction	A fraction is a part of a whole. A fraction is written mathematically as one value on top of another, separated by a fraction bar. It is also called a rational number.

Additional Resources

Interactive Element

Video: Inductive Reasoning

Activities: Conjectures and Counterexamples Discussion Questions

Study Aids: Types of Reasoning Study Guide

Practice: Conjectures and Counterexamples

Real World: Conjectures And Counterexamples