If three or more parallel lines are intersected by two transversals, prove that the intercepts made by them on the transversals are proportional.

$ABCD$ is a parallelogram and $P$ is a point on the side $BC$. $DP$ when produced meets $AB$ produced at $L$, Prove that $\frac{DP}{LP}=\frac{DC}{BL}$

$ABCD$ is a parallelogram and $P$ is a point on the side $BC$. $DP$ when produced meets $AB$ produced at $L$, Prove that $\frac{DL}{DP}=\frac{AL}{DC}$

In a triangle $△ABC$, $D$ and $E$ are points on the sides $AB$ and $AC$ respectively, such that $AD\times EC=AE\times DB$. Prove that $DE\parallel BC$  

The side $BC$ of a $△ABC$  is bisected at $D$, $O$ is any point on $AD$. $BO$ and $CO$ produced meet $AC$ and $AB$ at $E$ and $F$ respectively and $AD$ is produced to $X$ so that $D$ is the midpoint of $OX$. Prove that $AO:AX=AF:AB$ and show that $FE\parallel BC$.