sin＾4θ +cos^4θ=5/9,(sin＾2θ +cos^2θ)^2-2 sin＾2θ cos^2θ=5/9,1-2 sin＾2θ cos^2θ=5/9,2 sin＾2θ cos^2θ=4/9,4 sin＾2θ cos^2θ=8/9,即(2sinθcosθ) ^2=8/9,(sin2θ)^2=8/9,sin2θ=±2√2/3.
