As we know that exponential and cosine functions are continuous and differentiable on R.
Let us find the values of the function at an extreme
Here, f'(-π/2) = f(π/2), therefore there exist a c ∈ (-π/2,π/2) such that f'(c) = 0.
Let us find the derivative of f(x)
Here, f'(c) = 0,
Thus, Rolle'* theorem is verified.