a+b=10,c=84(a+b)=5c由正弦定理得4(sinA+sinB)=5sinC=5sin(A+B)8sin(A+B)/2*cos(A-B)/2=10sin(A+B)/2*cos(A+B)/24cos(A-B)/2=5cos(A+B)/24cos(A/2)cos(B/2)+4sin(A/2)sin(B/2)=5cos(A/2)cos(B/2)-5sin(A/2)sin(B/2)9...