数学题三角函数已知2sin(3π+θ)=cos(π+θ),求2sin^2 θ+3sinθcosθ-cos^2 θ

2sin(3π+θ)=cos(π+θ)
2sinθ=cosθ
tanθ=1/2
2sin^2 θ+3sinθcosθ-cos^2 θ 2sin^2 θ+3sinθcosθ-cos^2 θ
___________________________= ____________________________=
1 sin^2 θ +cos^2 θ
分子分母同除以cos^2 θ
2tan^2θ+3tanθ-1
________________ 将tanθ=1/2带入
tan^2θ+1
=4/5