f'(0)
＝lim(x→0) [f(x)－f(0)]/x 替换x为-t
＝lim(t→0) [f(-t)－f(0)]/(-t)
＝lim(t→0) [f(t)－f(0)]/(-t)
＝－lim(t→0) [f(t)－f(0)]/t
＝－f'(0)
所以,f'(0)＝0