1.5 cosBcosC(tanaBtanC- 1)= 5(sinb sinc-cosBcosC)=-5 cos(b+ C)= 5 cosA =-3，cosA=-3/5，sinA = 4/5；

2.a/sinA=b/sinB，sinB=b/a*sinA=5/4√2*4/5=√2/2，B=45，

cosc =-cos(A+B)= Sina sinb-Cosacosb = 4/5 *√2/2+3/5 *√2/2 = 7√2/ 10，C 2 = A 2+B 2-2 ABCOSC = 654338。