Given: A ║gm ABCD in which AB is the base and AL is the corresponding height.
To prove : Area ( ║gm ABCD) = AB × AL.
Construction: Draw BM ⊥ DC so that rectangle ABML is formed.
Proof: ║gm ABCD and rectangle ABML are on the same base AB and between the same parallel lines AB and LC.
∴ ar( ║gm ABCD) = ar(rectangle ABML) = AB × AL.
∴ area of a ║gm = base × height
