A good geometry problem

Geometry Level 5

In the diagram below, angle A B C ABC is a right angle. Point D D is on B C \overline{BC} , and A D \overline{AD} bisects angle C A B CAB . Points E E and F F are on A B \overline{AB} and A C \overline{AC} , respectively, so that A E = 3 AE = 3 and A F = 10 AF=10 . Given that E B = 9 EB = 9 and F C = 27 FC = 27 , find the integer closest to the area of quadrilateral D C F G DCFG .


The answer is 148.

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1 solution

Priyanshu Mishra
Jan 5, 2016

By the Pythagorean Theorem, B C = 35 BC = 35 . Letting B D = x BD = x we can use the angle bisector theorem on triangle A B C ABC to get x / 12 = ( 35 x ) / 37 x/12 = (35 - x)/37 , and solving gives B D = 60 / 7 BD = 60/7 and D C = 185 / 7 DC=185/7 .

The area of triangle A G F AGF is 10 / 3 10/3 that of triangle A E G AEG , since they share a common side and angle, so the area of triangle A G F AGF is 10 / 13 10/13 the area of triangle A E F AEF .

Since the area of a triangle is a b sin C 2 \large\ \frac{ab\sin{C}}2 , the area of A E F AEF is 525 / 37 525/37 and the area of A G F AGF is 5250 / 481 5250/481 .

The area of triangle A B D ABD is 360 / 7 360/7 , and the area of the entire triangle A B C ABC is 210 210 . Subtracting the areas of A B D ABD and A G F AGF from 210 210 and finding the closest integer gives 148 \boxed{148} as the answer.

Used basically the same approach. Should it really be level 5 !! I would put it in level 3.

Niranjan Khanderia - 5 years, 5 months ago

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This question is lengthy though easy ,I agree with your point.

Akshay Yadav - 5 years, 5 months ago

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I did this with co-ordinate geometry, it was kinda long.

Akshat Sharda - 5 years, 5 months ago

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@Akshat Sharda Same, it was the only method I could think of at the time, wasn't fun :P

Tai Ching Kan - 5 years, 5 months ago

That decision is taken by moderator. We can't do any changes we want.

Priyanshu Mishra - 5 years, 5 months ago

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I agree. I just gave my view.

Niranjan Khanderia - 5 years, 5 months ago

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@Niranjan Khanderia No problem. You can say what you want.

Priyanshu Mishra - 5 years, 5 months ago

Exactly same way.

Kushagra Sahni - 5 years, 5 months ago

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