这么写其实好点 f(0)＝f(0)xf(0),同时除以f(0)(f(0≠0，可以除))，就得f(0)等于1
f(0+0)=f(0)*f(0)f(0)=f(0)*f(0)f(0)-f(0)*f(0)=0f(0)[1-f(0)]=0
