f(x-1/x)=x^2+1/x^2=(x-1/x)²+2设x-1/x=t则f(t)=t²+2所以f(x)=x²+2
A-B=(cosa-cosb, sina-sinb)|A-B|^2=(cosa-cosb)^2+(sina-sinb)^2=2-2(cosacosb+sinasinb)=4/5cosacosb+sinasinb=3/5cos(a-b)=cosacosb+sinasinb=3/5-π/2
