Gergonne's Theorem

Let $\triangle ABC$ be a triangle and points $M$ , $N$ , and $P$ to be points on sides $BC, AC,$ and $AB$ respectively such that lines $AM, BN,$ and $CP$ are concurrent at point $O.$ Then, $\frac{OM}{AM} + \frac{ON}{BN} + \frac{OP}{CP} = 1$ .