(a²－b²)[sinC]=(a²＋b²)[sinAcosB－cosAsinB]
c(a²－b²)=(a²＋b²){a×[(a²＋c²－b²)/(2ac)]－b×[(b²＋c²－a²)/(2bc)]}
c²(a²－b²)=(a²－b²)(a²＋b²)
a²－b²=0或c²=a²＋b²