Given a minor sector $AOB$ (Here minor means that $ \angle AOB <90$). $O$ is the centre , chose a point $C$ on arc $AB$ ,Let $P$ be a point on segment $OC$ , join $AP$ , $BP$ , draw a line through $B$ parallel to $AP$ , the line meet $OC$ at point $Q$ ,join $AQ$ . Prove that the area of polygon $AQPBO$ does not change when points $P,C$ move .