Consider parallelogram PQRS and parallelogram ABRS.
As these lie on the same base SR and are between the same parallel lines PB and SR,
ar(PQRS) = ar(ABRS) .....(1)
Consider ΔAXS and parallelogram ABRS.
As these lie on the same base AS and are between the same parallel lines AS and BR,
∴ar(ΔAXS)=12ar(ABRS)...(2)
From equations(1) and(2), we obtain
ar(ΔAXS)=12ar(PQRS)