(a+b+c)(sinA+sinB-sinC)=3asinB即(a+b+c)(a+b-c)=3aba²+2ab+b²-c²=3aba²+b²-c²=abcosC=(a²+b²-c²)/2ab=1/2C=60cosAcosB-sinAsinB>0cos(A+B)>0cos(180-C)>0-cosC>0cosC<0钝角三角形