f(sin(x/2))=1+2[cos(x/2)]^2-1=2[cos(x/2)]^2=2(1-[sin(x/2)]^2) 令sin(x/2)=t,则f(t)=2(1-t^2) 所以f(cosx)=2(1-[cosx]^2)=2(sinx)^2
