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$ P $ and $ Q $ are respectively the mid-points of sides $ \mathrm{AB} $ and $ \mathrm{BC} $ of a triangle $ \mathrm{ABC} $ and $ \mathrm{K} $ is the mid-point of $ \mathrm{AP} $, show that
(i) $ \operatorname{ar}(\mathrm{PRQ})=\frac{1}{2} \operatorname{ar}(\mathrm{ARC}) $
(ii) ar $ (\mathrm{RQC})=\frac{3}{8} $ ar $ (\mathrm{ABC}) $
(iii) ar $ (\mathrm{PBQ})=\operatorname{ar}(\mathrm{ARC}) $
Given:
\( P \) and \( Q \) are respectively the mid-points of sides \( \mathrm{AB} \) and \( \mathrm{BC} \) of a triangle \( \mathrm{ABC} \) and \( \mathrm{K} \) is the mid-point of \( \mathrm{AP} \)
To do:
We have to show that
(i) \( \operatorname{ar}(\mathrm{PRQ})=\frac{1}{2} \operatorname{ar}(\mathrm{ARC}) \)(ii) ar \( (\mathrm{RQC})=\frac{3}{8} \) ar \( (\mathrm{ABC}) \)
(iii) ar \( (\mathrm{PBQ})=\operatorname{ar}(\mathrm{ARC}) \)
Solution:
We know that,
The median of a triangle divides it into two triangles of equal area.
$PC$ is the median of $\triangle ABC$.
$ar (\triangle BPC) = ar (\triangle APC)$……….(i)
$RC$ is the median of $\triangle APC$.
$ar (\triangle ARC) = \frac{1}{2}ar (\triangle APC)$……….(ii)
$PQ$ is the median of $\triangle BPC$.
This implies,
$ar (\triangle PQC) = \frac{1}{2}ar (\triangle\triangle BPC)$……….(iii)
From (i) and (iii), we get,
$ar (\triangle PQC) = \frac{1}{2} ar (\triangle APC)$……….(iv)
From (ii) and (iv), we get,
$ar (\triangle PQC) = ar (\triangle ARC)$……….(v)
$P$ and $Q$ are the mid-points of $AB$ and $BC$ respectively.
$PQ \| AC$
$PA = \frac{1}{2}AC$
The triangles between the same parallels are equal in area.
This implies,
$ar (\triangle APQ) = ar (\triangle PQC)$……….(vi)
From (v) and (vi), we get,
$ar (\triangle APQ) = ar (\triangle ARC)$……….(vii)
$R$ is the mid-point of $AP$.
$RQ$ is the median of $\triangle APQ$.
This implies,
$ar (\triangle PRQ) = \frac{1}{2}ar (\triangle APQ)$……….(viii)
From (vii) and (viii), we get,
$ar (\triangle PRQ) = \frac{1}{2}ar (\triangle ARC)$
(ii) $PQ$ is the median of $\triangle BPC$
$ar (\triangle PQC) = \frac{1}{2} ar (\triangle BPC)$
$= \frac{1}{2}\times[\frac{1}{2}ar (\triangle ABC)]$
$= \frac{1}{4}ar (\triangle ABC)$……….(ix)
$ar (\triangle PRC) = \frac{1}{2} ar (\triangle APC)$ [From (iv)]
$ar (\triangle PRC) = \frac{1}{2}\times[\frac{1}{2}ar (\triangle ABC)]$
$= \frac{1}{4}ar (\triangle ABC)$……….(x)
Adding (ix) and (x), we get,
$ar (\triangle PQC) + ar (\triangle PRC) =(\frac{1}{4}+\frac{1}{4})ar (\triangle ABC)$
$ar (PQCR) = \frac{1}{2} ar (\triangle ABC)$……….(xi)
Subtracting $ar(\triangle PRQ)$ from both sides,
$ar (PQCR)-ar (\triangle PRQ) = \frac{1}{2}ar (\triangle ABC)-ar (\triangle PRQ)$
$ar (\triangle RQC) = \frac{1}{2}ar (\triangle ABC) - \frac{1}{2} ar (\triangle ARC)$ (Proved)
$ar (\triangle ARC) = \frac{1}{2} ar (\triangle ABC) -\frac{1}{2} \times [\frac{1}{2}ar (\triangle APC)]$
$ar (\triangle RQC) = \frac{1}{2}ar (\triangle ABC)-(\frac{1}{4})ar (\triangle APC)$
$ar (\triangle RQC) = \frac{1}{2}ar (\triangle ABC)-\frac{1}{4} \times [\frac{1}{2}ar (\triangle ABC)]$ (Since $PC$ is the median of $\triangle ABC$)
$ar (\triangle RQC) = \frac{1}{2}ar (\triangle ABC)-\frac{1}{8}ar (\triangle ABC)$
$ar (\triangle RQC) = [(\frac{1}{2}-(\frac{1}{8})]ar (\triangle ABC)$
$ar (\triangle RQC) = \frac{3}{8}ar (\triangle ABC)$
(iii) $ar (\triangle PRQ) = \frac{1}{2}ar (\triangle ARC)$
$2ar (\triangle PRQ) = ar (\triangle ARC)$……………..(xii)
$ar (\triangle PRQ) = \frac{1}{2}ar (\triangle APQ)$ ($RQ$ is the median of APQ$)……….(xiii)
$ar (\triangle APQ) = ar (\triangle PQC)$
From (xiii) and (xiv), we get,
$ar (\triangle PRQ) = \frac{1}{2} ar (\triangle PQC)$……….(xv)
$ar (\triangle BPQ) = ar (\triangle PQC)$ ($PQ$ is the median of \triangle BPC$)……….(xvi)
From (xv) and (xvi), we get,
$ar (\triangle PRQ) =\frac{1}{2}ar (\triangle BPQ)$……….(xvii)
From (xii) and (xvii), we get,
$2\times(\frac{1}{2})ar(\triangle BPQ)= ar (\triangle ARC)$
$ar (\triangle BPQ) = ar (\triangle ARC)$
Hence Proved.
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