4.
In Fig. 9.16, P is a point in the interior of a
parallelogram ABCD. Show that
ma (APB) + 2 (PCD)= *(ABCD)
G (APD) +ar (PBC)=2(APB) + 2 (PCD)
(Hint: Through P. draw a line parallel to AB.)
Fig. 9.16
Answers
Given: ABCD is a parallelogram
So, AB||CD & AD|| BC
To show:
(i) ar (APB) + ar (PCD) = ar(ABCD)
(ii) ar (APD) + ar (PBC) = ar (APB) + ar (PCD)
Proof:
(i)
Through the point P ,draw GH parallel to AB.
In a parallelogram,
AB || GH (by construction) — (i)
Thus,
AD || BC ⇒ AG || BH — (ii)
From equations (i) and (ii),
ABHG is a parallelogram.
Now,
In ΔAPB and parallelogram ABHG are lying on the same base AB and between the same parallel lines AB and GH.
∴ ar(ΔAPB) = 1/2 ar(ABHG) — (iii)
also,
In ΔPCD and parallelogram CDGH are lying on the same base CD and between the same parallel lines CD and GH.
∴ ar(ΔPCD) = 1/2 ar(CDGH) — (iv
Adding equations (iii) and (iv),
ar(ΔAPB) + ar(ΔPCD) = 1/2 {ar(ABHG) + ar(CDGH)}
ar(APB) + ar(PCD) = 1/2 ar(ABCD)
(ii)
A line EF is drawn parallel to AD passing through P.
In a parallelogram,
AD || EF (by construction) — (i)
Thus,
AB || CD ⇒ AE || DF — (ii)
From equations (i) and (ii),
AEDF is a parallelogram.
Now,
In ΔAPD and parallelogram AEFD are lying on the same base AD and between the same parallel lines AD and EF.
∴ ar(ΔAPD) = 1/2 ar(AEFD) — (iii)
also,
In ΔPBC and parallelogram BCFE are lying on the same base BC and between the same parallel lines BC and EF.
∴ ar(ΔPBC) = 1/2 ar(BCFE) — (iv)
Adding equations (iii) and (iv),
ar(ΔAPD) + ar(ΔPBC) = 1/2 {ar(AEFD) + ar(BCFE)}
ar(ΔAPD) + ar(ΔPBC) =1/2 ar(ABCD)
ar(APD) + ar(PBC) = ar(APB) + ar(PCD) ( From part i)
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