√3sinα+cosα=2sin(a+π/3)=2/3
所以sin(a+π/3)=1/3,cos(a-π/6)=cos(π/6-a)=sin[π/2-(a+π/3)]=正负2（根号2）/3
cos（2a-π/3）=2[cos(a-π/6)]^2-1=7/9