1 f(x)=f(-x)
f'(x)=-f'(-x)
f'(0)=-f'(0)
f'(0)=0
2 拉格朗日中值定理
arctanx2-arctanx1=(1/(1+x^2))(x2-x1)
|arctanx2-arctanx1|=(1/(1+x^2))|(x2-x1)|