b=β
cos(2a+b)=cos(a+b)cosa-sinasin(a+b)
=1/3-sinasin(a+b)
cos(a+b)=1===>a+b=2kπ
b=2kπ-a
cosb=cos(2kπ-a)=cosa=1/3
sinb=-sina
cos2a=cos(4kπ-2b)=cos2b=2cos^2b-1=-7/9
1-sin^2a=1/9
sin^2a=8/9
sina=±2√2/3
sinasinb=-sin^2a=-8/9
即sinasinb与a或b所在象限无关
cos(2a+b)=cos2acosb-sin2asinb
=-(7/9)*(1/3)-2sinacosasinb
=-7/27+2*(8/9)*(1/3)
=(16-7)/27
=9/27
=1/3