(sina-cosa)^2/sin^4a+2sin^2acos^2a+cos^4a=1-sin2a

(sina-cosa)^2/sin^4a+2sin^2acos^2a+cos^4a=1-sin2a(sina-cosa)^2 = Sin²α -2SinαCosα + Cos²α = 1 - Sin2αsin^4a+2sin^2acos^2a+cos^4a =(Sin²2α + Cos²2α)²=1