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CD and GH are respectively the bisectors of $\angle ACB$ and $\angle EGF$ such that D and H lie on sides AB and FE of $∆ABC$ and $∆EFG$ respectively. If $∆ABC \sim ∆FEG$, show that
(i) $ \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} $>
(ii) $ \triangle \mathrm{DCB} \sim \triangle \mathrm{HGE} $>
(iii) $ \triangle \mathrm{DCA} \sim \triangle \mathrm{HGF} $
Given:
CD and GH are respectively the bisectors of $\angle ACB$ and $\angle EGF$ such that D and H lie on sides AB and FE of $∆ABC$ and $∆EFG$ respectively.
$∆ABC \sim ∆FEG$
To do:
We have to show that
(i) \( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} \)
(ii) \( \triangle \mathrm{DCB} \sim \triangle \mathrm{HGE} \)
(iii) \( \triangle \mathrm{DCA} \sim \triangle \mathrm{HGF} \)
Solution:
(i)
$\triangle ABC$ and $\triangle FEG$
This implies,
$\angle A=\angle F$
$\angle B=\angle E$
$\angle C=\angle G$
$\frac{AB}{FE}=\frac{BC}{EG}=\frac{AC}{FG}$
In $\triangle ACD$ and $\triangle FGH$,
$\angle A=\angle F$
$\angle 1=\angle 2$ (Since $\frac{1}{2}\angle C=\frac{1}{2}\angle G$)
Therefore, by AA criterion,
$\triangle ACD \sim \triangle FGH$
This implies,
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} \)
Hence proved.
(ii) $\triangle ACD \sim \triangle FGH$
This implies,
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} \)
\( \frac{\mathrm{AC}}{\mathrm{FG}}=\frac{\mathrm{BC}}{\mathrm{EG}} \)
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{BC}}{\mathrm{EG}} \)
In $\triangle BCD$ and $\triangle EGH$,
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{BC}}{\mathrm{EG}} \)
$\angle 3=\angle 4$ (Since $\frac{1}{2}\angle C=\frac{1}{2}\angle G$)
Therefore, by SAS criterion,
$\triangle DCB \sim \triangle HGE$
Hence proved.
(iii) $\triangle ACD \sim \triangle FGH$
This implies,
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} \)
\( \frac{\mathrm{AC}}{\mathrm{FG}}=\frac{\mathrm{BC}}{\mathrm{EG}} \)
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{BC}}{\mathrm{EG}} \)
In $\triangle DCA$ and $\triangle HGF$,
\( \frac{\mathrm{CD}}{\mathrm{GH}}=\frac{\mathrm{AC}}{\mathrm{FG}} \)
$\angle 1=\angle 2$ (Since $\frac{1}{2}\angle C=\frac{1}{2}\angle G$)
Therefore, by SAS criterion,
$\triangle DCA \sim \triangle HGF$
Hence proved.