tanα=-1/3 则sinα^2+2sinαcosα-3cosα^2=?

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tanα=-1/3 则sinα^2+2sinαcosα-3cosα^2=?
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tanα=-1/3 则sinα^2+2sinαcosα-3cosα^2=?
tanα=-1/3 则sinα^2+2sinαcosα-3cosα^2=?

tanα=-1/3 则sinα^2+2sinαcosα-3cosα^2=?
tanα=-1/3 则tanα^2=1/9
(sinα^2+2sinαcosα-3cosα^2)/cosα^2
=1/tanα^2-3+2xtanα
=9-3+(-2/3)
=10/3
sinα/cosα=-1/3
sinα^2+cosα^2=1
cosα^2=9/10
sinα^2+2sinαcosα-3cosα^2=3