因为sin2θ=a=2sinθcosθ=a cosθsinθ=a/2 sin（π/4-θ）=√2/2cosθ-√2/2sinθ 所以sin^2（π/4-θ）=1/2(cosθ-sinθ)^2=1/2(1-2cosθsinθ) 又因为;5π/4