已知tan（3π-α)=2,则sin(-3π+α)

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tan（3π-a)=2 .tana=-2,sin(-3π+a)=-sina,cos(π-a）=-cosa,cos（π+a）=-cosa,sin(-a)=-sina
tana=-2 ,sina=-2cosa,sin^2a=4cos^2a ,sin^2a=1-4sin^2a,sina=±√5/5
sin(-3π+a)+cos(π-a）/-cos（π+a）+sin(-a)
=-sina-cosa/cosa-sina=-2sina-1=±2√5/5-1

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