由正弦定理,a /sin A =b /sin B =c /sin C,则 sin B =b (sin A) /a,c =a (sin C) /sin A.又因为 a =3 √5,sin A =3/5,b=5,所以 sin B = √5 /5.因为 A为钝角,所以 B为锐角.所以 cos A = -√ [ 1 -(sin A)^2 ]= -4/5,...我算的是20-10根号3，不知道对不对