cos(pai-a)=-1/7,cos(a+b)=-11/14,求b
问题描述：
cos(pai-a)=-1/7,cos(a+b)=-11/14,求b
答
cos(pai-a)=-1/7cos(a)=1/7 sin(a)=4√3/7cos(a+b)=-11/14cos(a+b)=cos(a)cos(b)-sin(a)sin(b)=-11/14cos(b)/7-4√3sin(b)/7=-11/14cos(b)=4√3sin(b)-11/2sin(b)平方+cos(b)平方=1sin(b)=√3/2b=60