作业帮已知2sin(3π+θ)=cos(π+θ),求2sin^2 θ+3sinθcosθ-cos^2 θ
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已知2sin(3π+θ)=cos(π+θ),求2sin^2 θ+3sinθcosθ-cos^2 θ
已知2sin(3π+θ)=cos(π+θ),求2sin^2 θ+3sinθcosθ-cos^2 θ
已知2sin(3π+θ)=cos(π+θ),求2sin^2 θ+3sinθcosθ-cos^2 θ
由已知条件知：-2sinθ=-cosθ,即cosθ=2sinθ.
加上sin²θ+cos²θ=1.解得：sinθ=√5/5.
所以2sin²θ+3sinθcosθ-cos²θ
=2sin²θ+6sin²θ-4sin²θ
=4sin²θ
=4/5.
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