In $$ \Delta ABC ,\ AB = AC ,\ M $$ and $$N $$ are point on $$AB$$ and $$AC$$ such that $$BM = CN$$
$$BN$$ and $$CM$$ are joined.
In $$ \Delta AMC$$ and $$\Delta ANB $$
$$AB = AC$$ [given] ...(1)
$$BM = CN$$ [given] ...(2)
Subracting (2) from (1) we have
$$AB - BM - AC - CN$$
$$AM = AN$$