A technique often helpful in computing the derivatives of a differentiable function ƒ (x ); set g(x ) = log ƒ (x ) where ƒ (x ) ≠ 0, then g'(x ) = ƒ '(x )/ ƒ (x ), and if there is some other way to find g'(x ), then one also finds ƒ '(x ).