(tan a)^2=2(tan b)^2+1 (sina/cosa)^2=2(sinb/cosb)^2+1 1+(sina/cosa)^2=2(sinb/cosb)^2+2 1/(cosa)^2=2/(cosb)^2 (cosa)^2=(cosb)^2/2 1-(sina)^2=[1-(sinb)^2]/2 这样就有: (sinb)^2=2*(sina)^2-1
