Counterexample and reduction to absurdity are two methods to judge whether a proposition is true or false, but they are essentially different.

The so-called counterexample usually refers to an example used to illustrate that an example is not valid. A counterexample is a way to prove that a proposition is false, such as "the sum of two irrational numbers is irrational."

For example, if the diagonals of a convex polygon are all equal, then this convex polygon (). (a) Must be quadrilateral.

It must be a Pentagon.

(c) It is quadrangular or pentagonal.

(d) is a polygon with equal sides or equal internal angles.

We can determine the correctness of each option by giving counterexamples. Because a regular Pentagon is a polygon with equal diagonal lines, it is not a quadrilateral, so it can be denied.