KöMaL 2010 C1021

$P$ is a point on side $AC$, and $Q$ is a point on side $BC$ of triangle ABC . The line through $P$, parallel to $BC$ intersects $AB$ at $K$, and the line through $Q$, parallel to $A$C intersects $AB$ at $L$. Prove that if $PQ$ is parallel to $AB$ then $AK$=$BL$.