sinα+cosα=3分之根号3
则
2sinacosa
=(sina+cosa)^2-(sin^2a+cos^2a)
=1/3-1
=-2/3
所以
(cosa-sina)^2
=(cosa+sina)^2-4sinacosa
=1/3-2(-2/3)
=5/3
又因为0〈α〈π
所以π/2〈α〈π（2sinacosa=_2/3