A triangle ABC in which AB=AC, M is a point on AB and N is a point on AC such that if BM=CN then AM=AN.
Since M is the midpoint of AB and N is the midpoint of AC, we have
AB=AM+MB and AB=AN+NC
So,
AB=AC[Given]AM+BM=AN+CMAM+BM=AN+BM[Given BM=CN]AM=AN
Hence proved, AM=AN.