f'(x) = lim(h→0)[f(x+h)-f(x)]/h
　　= lim(h→0)[sin(x+h)-sinx]/h
　　= lim(h→0)2[sin(h/2)]cos(x+h/2)/h
　　= lim(h→0)cos(x+h/2)*lim(h→0)[sin(h/2)/(h/2)]
　　= cosx