sin^4θ+cos^4θ=7/8
(sin^2θ+cos^2θ)^2-2sin^2θcos^2θ=7/8
1-2sin^2θcos^2θ=7/8
1-1/2(sin2θ)^2=7/8
sin2θ=1/4或sin2θ=－1/4