已知sina+cosa=-1/5,0

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已知sina+cosa=-1/5,0
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已知sina+cosa=-1/5,0
已知sina+cosa=-1/5,0

已知sina+cosa=-1/5,0
sina+cosa=-1/5
(sina+cosa)^2=1/25
2sinacosa=-24/25
sinacosa=-12/25
sina+cosa=-1/5
可解得
sina=3/5
cosa=-4/5
cos(2a-π/4)
=cos2acosπ/4+sin2asinπ/4
=cos2a/2+sin2a/2
=[(cosa)^2-(sina)^2]/2 + sinacosa
=-17/25