Replacing x with -X can skillfully get another formula, and at the same time, eliminating f(-X) can easily get the analytical formula of f(X). In short, it is to change the original formula. In fact, as long as the problem can be solved quickly, x can be set to any value.

What is involved here is actually a substitution idea, so I don't need to go into details. If you know the idea of substitution, you can practice more questions. I wish you progress in your study and hope to adopt it!