若tanθ=2,求3cos^2-2sinθcosθ.
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若tanθ=2,求3cos^2-2sinθcosθ.
若tanθ=2,求3cos^2-2sinθcosθ.
若tanθ=2,求3cos^2-2sinθcosθ.
tanθ=2
sinθ/cosθ=2
sinθ=2cosθ 1
1式平方
sin^2 θ=1-cos^2 θ=4cos^2 θ
cos^2 θ=1/5
3cos^2 θ-2sinθcosθ
=3cos^2 θ-2*2cosθ*cosθ
=3cos^2 θ-4cos^2 θ
=-cos^2 θ=-1/5