a属于(0,派),且cosa+sina=-1/3,所以a属于(3派/4,派)
2a属于(3派/2,2派)
cos2a > 0
(cosa+sina)^2 = 1 + 2sinacosa = 1+sin2a = 1/9
sin2a = -8/9
cos2a = √[1-(8/9)^2] = √17/9