f‘(x)=lim(f(x+h)-f(h))/h
f‘(-x)=lim(f(-x+h)-f(h))/h f(x)为奇函数,f(-x+h)=- f(x-h)
=lim(-f(x-h)-f(h))/h
=lim(f(x-h)-f(h))/(-h)
=f‘(x)
所以：f’(x)为偶函数