已知cosa=4/5,cos(a+b)=3/5,且a,b均为锐角0°知0°∴sina=3/5,sin(a+b)=4/5sinb=sin[(a+b)-a]=sin(a+b)cosa-cos(a+b)sina=(4/5)×(4/5)-(3/5)×(3/5)=7/25
sina=3/5，cos（a+b）=cosacosb-sinasinb，令sinb=x代入解方程可得
