Find the derivative of `cos x` at x=0.

Explanation

Let `f(x)=cos x`
`therefore f'(0)=underset(hrarr0)"lim"(f(0+h)-f(0))/(h)`
`=underset(hrarr0)"lim"(-(1-cos h))/(h)`
`=underset(hrarr0)"lim"(-2sin^(2)(h)/(2))/(h)`
`=underset(hrarr0)"lim"(-sin(h)/(2))/((h)/(2)).sin(h)/(2)`
`=-(1).0=0`.