cos2a=3/5(sin2a)^2= (1-(cos2a)^2 = 16/25cos^2a - sin^2a = 3/5(cos^2a - sin^2a )^2 = 9/25(cos^2a - sin^2a )^2 = 9/25sin^4*a+cos^4*a-2(sinacosa)^2 = 9/25sin^4*a+cos^4*a-1/2(sin2a)^2 = 9/25sin^4*a+cos^4*...