f '(x) = xsinx-cosx for 0<x<4, f has a local maximum when x is approximately ?

Question

the choices between A. 0.9, B. 1.2, C. 2.3, D.3.4, and E. 3.7

Answer 1

Suppose x is a local maximum or minimum, and that f'(x) exists. Then it can be proved that f'(x)=0.

Take the derivative of f, then plug in different values and check the condition.