In the figure, $ABCD$ is a parallelogram and $E$ is the mid-point of side $BC$. If $DE$ and $AB$ when produced meet at $F$, prove that $AF = 2AB$.
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Given:
$ABCD$ is a parallelogram and $E$ is the mid-point of side $BC$.
$DE$ and $AB$ when produced meet at $F$.
To do:
We have to prove that $AF = 2AB$.
Solution:
In $\triangle CDE$ and $\triangle EBF$,
$\angle DEC = \angle BEF$ (Vertically opposite angles)
$CE = EB$ ($E$ is the mid point of $BC$)
$\angle DCE = \angle EBF$ (Alternate angles)
Therefore, by SAS axiom,
$\triangle CDE \cong \triangle EBF$
This implies,
$DC = BF$ (CPCT)
$AB = DC$ (Opposite sides of a parallelogram)
Therefore,
$AB = BF$
$AF = AB + BF$
$= AB + AB$
$= 2AB$
Hence, $AF = 2AB$.
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