已知a=（sina,cosa),b=(cosb,sinb),b+c=(2cosb,0),ab=1/2,ac=1/3,求c

=(cosb,sinb),b+c=(2cosb,0),
所以,c=(cosb,-sinb)
ab=sinacosb+cosasinb=sin(a+b)=1/2
ac=sinacosb-cosasinb=sin(a-b)=1/3
ab+ac=2sinacosb=5/6,sinacosb=5/12
ab-ac=2cosasinb=1/6,cosasinb=1/12
cos2(a+b)+tanacotb
=(1-2*(sin(a+b))^2)+sinacosb/cosasinb
=(1-2*(1/2)^2)+(5/12)/(1/12)
=(1-1/2)+5
=11/2