已知cos（a-π/6）+sina=（4*根号3）/5,则sin（a-7π/6）的值是

cos（a-π/6）+sina=cosacosπ/6+sinasinπ/6+sina=cosa*(根号3）/2+sina*1/2+sina=cosa*(根号3）/2+sina*3/2=根号3*(cosa*1/2+sina*(根号3)/2=根号3*sin(π/6+a)=（4*根号3）/5所以sin(π/6+a)=4/5,cos(π/6+a)=±1...