Answer
See below.
Work Step by Step
1. $cos(x+h)=cos(x)cos(h)-sin(x)sin(h)$
2. $\frac{f(x_h)-f(x)}{h}=\frac{cos(x)cos(h)-sin(x)sin(h)-cos(x)}{h}
=\frac{-sin(x)sin(h)-cos(x)+cos(x)cos(h)}{h}
=\frac{-sin(x)sin(h)-cos(x)(1-cos(h))}{h}
=\frac{-sin(x)sin(h)}{h}-\frac{cos(x)(1-cos(h))}{h}$
