14sinθcosθ=5sin2φ 5cos2φ=7-7cos2θ=14sin^2 θ两式相除
得sinθ/cosθ=cos2φ/sin2φ cosθcos2φ-sinθsin2φ=0
cos(θ+2φ)=0 锐角θ,φ θ+2φ∈(0,3π/2) 所以θ+2φ=π/2