sin(π-θ)-cos(π+θ)=1/5
所以sina+cosa=1/5
1+2sinacosa=1/25
sinacosa=-12/25
sin(π-θ)=sinθ,cos(π+θ)=-cosθ,
sin(π-θ)-cos(π+θ)=1/5,
得sinθ+cosθ=1/5,两边平方,
sin²θ+cos²θ+2sinθcosθ=1/25,
sin²θ+cos²θ=1,则sinθcosθ=-12/25
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