Hi, could someone explain to me how to use a counterexample in trig to prove whether something is either an identity or equation?

Question

For ex. how would you do that with this question?

sec^2x-tan^2x= cos x

Answer 1

A counterexample is simply any instance that disproves the statement.

For example, try using x = pi/4. Note that this is NOT the only correct answer.

Answer 2

Would I substitute the same number to both sides?