1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx

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1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx1-2sinxcosx/cos2x-sin2x=1-

1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx
1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx

1-2sinxcosx/cos2x-sin2x=1-tanx/1+tanx
左边=(sin²x+cos²x-2sinxcosx)/(cos²x-sin²x)
=(cosx-sinx)²/(cosx+sinx)(cosx-sinx)
=(cosx-sinx)/(cosx+sinx)
上下除以cosx
且 sinx/cosx=tanx
所以左边=(1-tanx)/(1+tanx)=右边
命题得证

b(n+1)/bn
=-2a(n+1)÷(-2an)
=a(n+1)/an
=q
选A
(-2)△5即A=-2,B=5
所以(-2)△5=-2×5+5-(-2)
=-10+5+2
=-3