Think about two folds

Geometry Level 3

In A B C \triangle ABC , B E BE is a median and O O is the midpoint of B E BE . The line joining A A and O O meets B C BC at D D . Find the ratio of the lengths A O : O D AO : OD .


The answer is 3.000.

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3 solutions

Tapas Mazumdar
Mar 20, 2017

Draw E F EF parallel to A D AD as shown.

It can be easily shown that Δ B O D \Delta BOD and Δ B E F \Delta BEF are similar triangles. Thus by the basic proportionality theorem, we have

B O B E = O D E F 1 2 = O D E F E F = 2 O D \begin{aligned} & \dfrac{BO}{BE} &=& \dfrac{OD}{EF} \\ \\ \implies & \dfrac 12 &=& \dfrac{OD}{EF} \\ \\ \implies & \color{#3D99F6}{EF} &=& \color{#3D99F6}{2 OD} \end{aligned}

Also, Δ C E F \Delta CEF and Δ C A D \Delta CAD are similar. Thus by BPT,

C E C A = E F A D 1 2 = E F A D A D = 2 E F A O + O D = 4 O D A O = 3 O D A O O D = 3 1 \begin{aligned} & \dfrac{CE}{CA} &=& \dfrac{EF}{AD} \\ \\ \implies & \dfrac 12 &=& \dfrac{EF}{AD} \\ \\ \implies & AD &=& 2 \color{#3D99F6}{EF} \\ \\ \implies & AO+OD &=& 4OD \\ \\ \implies & AO &=& 3OD \\ \\ \implies & \dfrac{AO}{OD} &=& \boxed{\dfrac 31} \end{aligned}

Abdullah Ahmed
Mar 22, 2017

Add D , E D,E

Now let Δ \Delta A B E = x ABE=x and Δ \Delta B D O = a BDO=a

So Δ A B E = Δ B E C = x \Delta ABE=\Delta BEC=x

So Δ B D O = Δ D E O = a \Delta BDO=\Delta DEO=a

Then Δ A D E = Δ D E C = x + a \Delta ADE=\Delta DEC=x+a

So Δ B E C = x + 3 a = 2 x \Delta BEC=x+3a=2x

Then a = x / 3 a=x/3

So A O : O D = x : a = 1 : 1 / 3 = 3 : 1 AO:OD=x:a=1:1/3=3:1

Using Menelaus's Theorem in Δ BEC \Delta \text{BEC} with transversal AD \text{AD} ,

C A E A × E O B O × B D D C = 1 \displaystyle \dfrac{CA}{EA} \times \dfrac{EO}{BO} \times \dfrac{BD}{DC}=1

Using the given conditions this simplifies to DC = 2 BD \text{DC}=2\text{BD} , and similarly for Δ ACD \Delta \text{ACD} with transversal BE \text{BE} we get,

C B B D × D O O A × A E E C = 1 B D + D C B D = A O O D A O O D = 3 B D B D = 3 \displaystyle\begin{aligned} &\dfrac{CB}{BD}\times \dfrac{DO}{OA}\times \dfrac{AE}{EC} = 1 \\ &\dfrac{BD+DC}{BD} = \dfrac{AO}{OD} \\ & \dfrac{AO}{OD} = \dfrac{3BD}{BD}=\boxed{3} \end{aligned}

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