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Monday, January 21, 2019

Two externally tangent circle

Given : M is the midpoint of BC and as shown in figure .
Prove: BC = 2 AF
Solution

Given : M is the midpoint of BC and as shown in figure .
Prove: BC = 2 AF
\displaystyle \text{Proof}\ :\ \Delta \ BHA\sim \Delta \ BGC\ \ \left( {\because HA\parallel GC} \right)

\displaystyle \ \ \ \ \ \ \ \ \ \ \frac{{AB}}{{AH}}=\frac{{BC}}{{CG}}\ \ .......(1)

\displaystyle \ \ \ \ \ \ \ \ \ \ \text{In}\ \Delta \ BHA\ \ \text{and}\ \Delta \ FMA\ ,

\displaystyle \ \ \ \ \ \ \ \ \ \ \angle \ BHA=\angle \ FMA={{90}^{\circ }}

\displaystyle \ \ \ \ \ \ \ \ \ \ \angle \ BAH=\angle \ FAM\ \ \left( {\because \text{opposite}\ \angle \text{s}} \right)

\displaystyle \ \ \ \ \ \ \ \ \ \ \Delta \ BHA\sim \Delta \ FMA\ \ \left( {AA\ \text{Cor}:} \right)

\displaystyle \ \ \ \ \ \ \ \ \ \ \frac{{AB}}{{AH}}=\frac{{AF}}{{AM}}\ \ .......(2)

\displaystyle \ \ \ \ \ \ \ \ \ \ \text{From}\ (1)\ \text{and}\ (2),

\displaystyle \ \ \ \ \ \ \ \ \ \ \frac{{BC}}{{CG}}=\frac{{AF}}{{AM}}

\displaystyle \ \ \ \ \ \ \ \ \ \ \frac{{BC}}{{R-r}}=\frac{{AF}}{{\frac{1}{2}(R+r)-r}}

\displaystyle \ \ \ \ \ \ \ \ \ \ \ \frac{{BC}}{{R-r}}=\frac{{2AF}}{{R-r}}

\displaystyle \ \ \ \ \ \ \ \ \ \ \ \therefore \ BC=2AF


 

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