Suppose you were given a mathematical pattern like *h* = . What if you wanted to make an educated guess, or conjecture, about *h*? After completing this Concept, you'll be able to make such a guess and provide counterexamples that disprove incorrect guesses.

### Watch This

CK-12 Conjectures and Counterexamples

Watch the final part of this video.

### Guidance

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

#### Example A

Here’s an algebraic equation and a table of values for and .

1 | 0 | |

2 | 0 | |

3 | 0 |

After looking at the table, Pablo makes this conjecture:

The value of is 0 for any number .

Is this a true conjecture?

This is not a valid conjecture. If Pablo were to continue the table to , he would have see that

In this example is the counterexample.

#### Example B

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

From these examples, Arthur made this conjecture:

If a convex polygon has sides, then there are triangles formed when diagonals are drawn from any vertex of the polygon.

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

The conjecture appears to be correct. If Arthur draws other polygons, in every case he will be able to draw triangles if the polygon has 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 C

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

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

CK-12 Conjectures and Counterexamples

### Guided Practice

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.

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

2. Can you think of a counterexample?

**Answers:**

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

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

### Practice

Give a counterexample for each of the following statements.

- If is a whole number, then .
- All numbers that end in 1 are prime numbers.
- All positive fractions are between 0 and 1.
- Any three points that are coplanar are also collinear.
- All girls like ice cream.
- All high school students are in choir.
- For any angle there exists a complementary angle.
- All teenagers can drive.
- If is an integer, then .
- All equations have integer solutions.