证明：在三角形ABC中,角A,B,C的对边a,b,c,所以sin`A/sinC = a/c,sinB/sinC = b/c 因此（a^2-b^2)/c^2=[sin^2(A)-sin^2(B)]/sin^2(C) =[1/2(1-cos2A)-1/2（1-cos2B)]/sin^2(C) =1/2(cos2B-cos2A)/sin^2(C) =1/2[-2sin...