Triangular Madness

Geometry Level 4

Given the following:

  • Δ ( A B C ) = 12345 | \Delta(ABC) | = 12345
  • B D = D C , B E = E D , A P = P D BD=DC, BE=ED, AP=PD
  • G D F C , GD\parallel FC,

find the area of the shaded region.


The answer is 2469.

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3 solutions

Let X X be the point of intersection of the straight lnies A E AE adnd G D GD .
Let Y Y be the point of intersection of the straight lnies A E AE adnd F C FC .

A P = P D AP = PD and F C G D FC \| GD . By midpoint theorem, A Y = X Y AY=XY and A Y + X Y = A X = 2 X Y AY +XY = AX = 2XY .

B D = B C 2 = D C B E = E D = B C 4 E D D C = 1 2 \begin{aligned} BD &=& \frac{BC}2 = DC \\ BE&=& ED = \frac{BC}4 \\ \therefore \frac{ED}{DC} &=& \frac12 \end{aligned}

G D F C GD \| FC , so by basic proportionality theorem,

E D D C = E X X Y 1 2 = = E X X Y E X 2 X Y = 1 4 E X A X = 1 4 \begin{aligned} \frac{ED}{DC} &=& \frac{EX}{XY} \\ \frac12 &=& = \frac{EX}{XY} \\ \frac{EX}{2XY} &=& \frac14 \\ \frac{EX}{AX} &=& \frac14 \end{aligned}

A ( X E D ) A ( A X D ) = 1 4 \frac{A(\triangle XED)}{A(\triangle AXD)} = \frac14

Applying componendo

A ( A E D ) A ( ( A X D ) = 5 4 A ( A B C ) 4 A ( ( A X D ) = 5 4 A ( A B C ) A ( ( A X D ) = 5 A ( ( A X D ) = A ( A B C ) 5 = 12345 5 = 2469 . \frac{A(\triangle AED)}{A(\triangle (AXD) } =\frac54 \\ \frac{A(\triangle ABC)}{4 \cdot A(\triangle (AXD) } =\frac54 \\ \frac{A(\triangle ABC)}{ A(\triangle (AXD) } =5 \\ A(\triangle (AXD) = \frac{A(\triangle ABC)}5 = \frac{12345}5 =\boxed{2469} .

Nice page but there is a problem in your solution you have repeated one step two times. Have a close look

Vishwash Kumar ΓΞΩ - 4 years, 6 months ago

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i know GD || FC i have purposely done that so that i don't have to direct the reader back to the same statement

A Former Brilliant Member - 4 years, 6 months ago

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come on rmo preparation slack loun

Vishwash Kumar ΓΞΩ - 4 years, 6 months ago
Ahmad Saad
Dec 1, 2016

Michael Mendrin
Dec 13, 2016

Extend line E C EC to line E G EG such that D C = C G DC = CG , and draw line A G AG . Then triangle E D H EDH is similar to triangle E G A EGA , but in proportion of E D : E G = 1 : 5 ED:EG = 1:5 . Hence, the altitude of triangle E D H EDH is 1 5 \dfrac{1}{5} of altitude of triangle E G A EGA . Given that the area of triangle E D A EDA is 1 4 \dfrac{1}{4} of triangle B C A BCA , and that the area of triangle D A H DAH is 1 1 5 = 4 5 1-\dfrac{1}{5}=\dfrac{4}{5} of triangle E D A EDA , the area of triangle D A H DAH is 1 4 4 5 = 1 5 \dfrac{1}{4}\cdot \dfrac{4}{5} = \dfrac{1}{5} of triangle B G A BGA , so that the area is 1 5 12345 = 2469 \dfrac{1}{5}\cdot 12345=2469 .

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