Question

已知A,B均为锐角,SINA=5/13,COS(A+B)=3/5,求COSB的值

Answer 1

COS(A+B)=3/5 ===> sin(A+B)＝4/5

SINA=5/13 ===> cos(A)＝12/13

cos(B)＝cos(A+B－A)＝Cos(A+B)*Cos(A)－Sin(A+B)*Sin(A)

＝3/5 * 12/13 － 4/5 * 5/13

＝16/65