2sinB＝sinA＋sinC
2sin(180°-(A+C))=2sin(A+C)/2cos(A-C)/2
sin(A+C)=sin(A+C)/2cos(A-C)/2
2sin(A+C)/2*cos(A+C)/2=sin(A+C)/2cos(A-C)/2
2cos(A+C)/2=cos(A-C)/2