Find the Area

Geometry Level 5

A point P P is chosen in the interior of A B C \triangle ABC so that liners are drawn through P P parallel to the sides of A B C \triangle ABC , the resulting smaller triangles t 1 t_1 , t 2 t_2 , t 3 t_3 in A B C \triangle ABC have areas 4, 9 and 49, respectively. Find the area of A B C \triangle ABC .


The answer is 144.

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2 solutions

Abdullah Ahmed
Jun 29, 2016

Let the line through P P parallel to B C BC intersect A B AB , A C AC at D D , E E respectively. Again let the line through P P parallel to C A CA intersect B C BC , A B AB at F F , G G respectively.Finally let the line through P P parallel to A B AB intersect B C BC , C A CA at K K , L L respectively.

Assume that P K F \triangle PKF = t 1 t_1 , P E L \triangle PEL = t 2 t_2 and P D F \triangle PDF = t 3 t_3

Now, P K F \triangle PKF , L P E \triangle LPE , G D P \triangle GDP and A B C \triangle ABC are all similar triangle.

Also, A G P L AGPL , B D P K BDPK , C E P F CEPF are all parallelograms

Next, E C L E \frac{EC}{LE} = P E L E \frac{PE}{LE} = ( K P L ) ( P L E ) \sqrt{\frac{(KPL)}{(PLE)}} = 4 9 \sqrt{\frac{4}{9}} = 2 3 \frac{2}{3}

Similarly, A L L E \frac{AL}{LE} = 7 3 \frac{7}{3}

So, A C L E \frac{AC}{LE} = A L + L E + E C L E \frac{AL+LE+EC}{LE} = A L L E \frac{AL}{LE} + L E L E \frac{LE}{LE} + E C L E \frac{EC}{LE} = 4 4

So, ( A B C ) ( L P E ) \frac{(ABC)}{(LPE)} = A C L E 2 \frac{AC}{LE}^{2} = 16 16

Which implies that ( A B C ) (ABC) = 144 144

It could be generalized that if the areas are a^2, b^2,c^2 then the area of the triangle is (a+b+c)^2.

Aditya Kumar - 4 years, 11 months ago
Ujjwal Rane
Sep 11, 2016

Three child triangles similar to their parent Three child triangles similar to their parent

Here all the three triangles are similar. So any triplet of their corresponding sides will be in a ratio of 2:3:7 (square roots of their areas)

Let us take their bases to be 2x, 3x, 7x which gives the base of the parent triangle to be (2x + 3x + 7x = 12x. So the area of the parent triangle must be ( 2 + 3 + 7 ) 2 = 144 (2 + 3 + 7)^2 = \textbf {144}

when i faced this qsn first , i solved it just in the same way ...

Abdullah Ahmed - 4 years, 9 months ago

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