设x=cos2a,y=sin2a x^2-y^2=cos4a=1/5 x^2+y^2=1 x^2=3/5 y^2=2/5x^2=(cos2a)^2=[(cosa)^2-(sina)^2]^2=(sina)^4+(cosa)^4-2(sina)^2(cosa)^2(sina)^4+(cosa)^4=x^2+2(sina)^2(cosa)^2=x^2+1/2y^2=4/5