tan(π/4-a)=-2,sin2a=
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tan(π/4-a)=-2,sin2a=
tan(π/4-a)=-2,sin2a=
tan(π/4-a)=-2,sin2a=
tanπ/4=1
所以tan(π/4-a)=(tana-1)/(1+tana)=-2
tana=-1/3
sina/cosa=-1/3
cosa=-3sina
代入sin²a+cos²a=1
sin²a=1/10
sin2a=2sinacosa=2sina(-3sina)=-6sin²a=-3/5