由余弦定理CosC=(a^2+b^2-c^2)/2ab ,所以CosC=ab/2ab =1/2,故C=60°因为sin(A+B)=sinAconB+conAsinB,由2cosA=sin(A加B)/sinB得2cosAsinB=sinAconB+conAsinB,所以cosAsinB=sinAconBsinAconB-cosAsinB=sin(A-B)=0,所以...