sin^4a+cos^4a=1
(sin^2a+cos^2a)^2-2sin^2acos^2a=1
sinacosa=0
(sina+cosa)^2=sin^2a+cos^2a+2sinacosa=1+0=1
sina+cosa=1 or sina+cosa=-1sinacosa=0 这个为什么等于0啊sin^4a+cos^4a=1(sin^2a+cos^2a)^2-2sin^2acos^2a=11-2sin^2acos^2a=12(sinacosa)^2=1-1=0sinacosa=0