是cosx的幂级数展开式公式 f(x)=f(a)+f'(a)/1!*(x-a)+f''(a)/2!*(x-a)^2+f'''(a)/3!*(x-a)^3+...f(x)=cos(x)取a=0cos(x)的各阶导数如下cos(0)=1cos'(0)=-sin(0)=0cos''(0)=-sin'(0)=-cos(0)=-1cos'''(0)=-sin''(0)=-...