# (d) What is counterexample? Explain its use with the help of an example

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A counterexample is an example or instance that disproves or contradicts a statement, proposition, or conjecture. In other words, it is a specific case where the given statement fails to hold true, demonstrating that the statement is not universally valid.

The use of counterexamples is crucial in mathematical reasoning and logic to test the validity of conjectures or statements. If a statement is proven to be false in a particular case, it is considered invalid. Here's an example:

Statement: "All prime numbers are odd."

To disprove this statement, we can provide a counterexample, which is a prime number that is not odd. The number 2 is a prime number, but it is even. Therefore, the statement is false, and 2 serves as a counterexample.

In this case:

- Statement: "All prime numbers are odd."

- Counterexample: \(2\) (a prime number that is not odd).

By presenting a counterexample, we show that the statement doesn't hold true universally. Counterexamples play a crucial role in refining mathematical theories and identifying the limitations or exceptions to general statements. They help mathematicians and researchers understand the precise conditions under which a statement is valid and when it might fail.