正弦定理:sinA/a = sinB/b = sinC/c,得
sinA=asinC/c, sinB=bsinC/c
余弦定理:cosA=(b^2+c^2-a^2)/2bc, cosB=(a^2+c^2-b^2)/2ac
sin(A-B)=sinAcosB-cosAsinB = asinC/c * (a^2+c^2-b^2)/2ac - bsinC/c *(b^2+c^2-a^2)/2bc
= sinC * (a^2-b^2)/c^2
所以:(a^2-b^2)/c^2=sin(A-B)/sinC