In figure, ABCDE is a pentagon. A line through B parallel to AC meets DC produced at F. Show that
(i) ar(ΔACB) = ar(ΔACF)
(ii) ar(□AEDF) = ar(ABCDE).
To Prove: □ABCD is a trapezium.
Proof: ar(ΔAOD) = ar(ΔBOC)
⇒ ar(ΔAOD) + ar(ΔAOB)
= ar(ΔBOC) + ar(ΔAOB)
| Adding the same areas on both sides
⇒ ar(ΔABD) = ar(ΔABC)
But ΔABD amd ΔABC are on the same base
AB.
∴ ΔABD and ΔABC will have equal corresponding altitudes.
ΔABD and ΔABC will lie between the same parallels.
∴ AB || DC
∴ [□ABCD is a trapezium.
A quadrilateral is a trapezium if exactly one pair of opposite sides is parallel
Show that the area of a rhombus is half the product of the lengths of its diagonals.
Or
Prove that the area of a rhombus is equal to half the rectangle contained by its diagonals.