Sin2A=2SinA•CosA ,Cos2A = Cos^2 A--Sin^2 A 所以cos2a/(1+sin2a)=（cosa-sina）/（sina+cosa）又因为sin(a) = [2tan(a/2)] / {1+[tan(a/2)]^2} cos(a) = {1-[tan(a/2)]^2} / {1+[tan(a/2)]^2} 所以……...