Classic Geometry Problem

Geometry Level 5

In the figure shown, A B C D ABCD is an arbitrary rectangle. Point E E lies on side B C BC such that B E E C = 2 \frac{BE}{EC}=2 and point F F lies on C D CD such that C F F D = 2 \frac{CF}{FD}=2 . Segments A E AE and A C AC intersect F B FB at points X X and Y Y , respectively. The ratio F Y : Y X : X B FY:YX:XB can be expressed as a : b : c a:b:c where a a , b b , and c c are coprime, positive integers. Find a + b + c a+b+c .


The answer is 65.

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5 solutions

Ajit Athle
Apr 21, 2014

Assume that D:(0,0), A:(0,3q), B:(3p,3q) & C:(3p,0) -- a general rectangle where p. q are unknown. We find line BF to be: y=3qx/(2p) - 3q/2 and AC : x/(3p)+y/(3q)=1 which gives us Y to be: (9p/5, 6q/5). Similarly, determine X as (27p/13, 21q/13). We can now say that FY:YX:XB=(9/5 -1):(27/13 - 9/5)):(3 - 27/13) = 26:9:30

i've also done in the similar manner

Priyesh Pandey - 7 years, 1 month ago

I tried the problem using Menalaus theorem. Please explain why the theorem doesn't hold good for this question.

Shreyansh Vats - 7 years, 1 month ago

65..

Steve Shen - 7 years, 1 month ago

( link try to do this problem please

Sudhamsh Suraj - 4 years, 3 months ago

A coordinate free proof. By Tales theorem F Y Y B = F C A B = 2 3 . \frac{|FY|}{|YB|} = \frac{|FC|}{|AB|} = \frac{2}{3}. that is F B = 5 2 F Y |FB| = \frac{5}{2}|FY| .

Let G G be the intersection point of lines A X AX and C D CD . Then again by Tales we have C G D C = C G A B = C E E B = 1 2 \frac{|CG|}{|DC|} = \frac{|CG|}{|AB|}= \frac{|CE|}{|EB|} =\frac{1}{2} and F X X B = F G A B = F C + C G A B = 2 3 + 1 2 = 7 6 , \frac{|FX|}{|XB|} = \frac{|FG|}{|AB|} = \frac{|FC| + |CG|}{|AB|} = \frac{2}{3} + \frac{1}{2} = \frac{7}{6}, that is F B = 13 7 F X |FB| = \frac{13}{7}|FX| . Hence we have that F Y F X = F B F X F Y F B = 13 7 2 5 = 26 35 , \frac{|FY|}{|FX|} = \frac{|FB|}{|FX|}\,\frac{|FY|}{|FB|} = \frac{13}{7} \cdot \frac{2}{5} = \frac{26}{35}, and Y X F Y = 35 26 35 = 9 26 . \frac{|YX|}{|FY|} = \frac{35 - 26}{35} = \frac{9}{26}. Now 3 2 = Y B F Y = Y X + X B F Y = 9 26 + X B F Y , \frac{3}{2}=\frac{|YB|}{|FY|} = \frac{|YX| + |XB|}{|FY|} = \frac{9}{26} + \frac{|XB|}{|FY|}, hence X B F Y = 3 2 9 26 = 39 9 26 = 30 26 \frac{|XB|}{|FY|} = \frac{3}{2} - \frac{9}{26} = \frac{39 - 9}{26} = \frac{30}{26} so that X B : Y X : F Y = 30 : 9 : 26 |XB|:|YX|:|FY| = 30:9:26 . As G C D ( 26 , 9 , 30 ) = 1 GCD\,(26,9,30)=1 , we have that a + b + c = 65 a+b+c = 65 .


Missed it due to slip in addition!!!

Niranjan Khanderia - 4 years, 8 months ago

Thanks for the solution.

Sahil Nare - 6 years, 1 month ago
Finn Hulse
Apr 16, 2014

Because A B C D ABCD is an arbitrary rectangle, let's just assume that it's a square. From here, let's let the side length be 3 3 . Now, we can graph this. We'll let Point D D lie on the origin, and so on and so forth, such that B B is located at ( 3 , 3 ) (3, 3) . Now, let's find the line A E AE . We know that length E C EC is equal to 1 1 , so E E is the point ( 3 , 1 ) (3, 1) . Thus, the graph of the line is y = 2 3 x + 3 y=\frac{-2}{3}x+3 . Now let's find F B FB . Similarly, we can label the point F F to be ( 1 , 0 ) (1, 0) . The graph of this line is 3 2 x 3 2 \frac{3}{2}x-\frac{3}{2} . To find point X X , let's set these equations equal. Solving, we find that X X is at the point ( 27 5 , 33 5 ) (\frac{27}{5}, \frac{33}{5}) . We can likewise deduce that point Y Y is ( 9 5 , 6 5 ) (\frac{9}{5}, \frac{6}{5}) . Now, applying the distance formula, we find that the ratio is 26 : 9 : 30 26:9:30 , and thus the desired answer is 26 + 9 + 30 = 65 26+9+30=\boxed{65} . And we're done! This is a really bad approach, the calculations are messy, it takes lots of time, etc., so try to find a cool way to tackle the problem. :D

finn, just write a good solution :) ppl here like me are not as good as you and calvin are. we need to know not only the answer, but also solution. it doesnt help to grow knowledge without knowing new approaches.

hope you post a solution soon :)

Nisarg Thakkar - 7 years, 1 month ago

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Yeah, I guess.

Finn Hulse - 7 years, 1 month ago

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thanx :)

Nisarg Thakkar - 7 years, 1 month ago

i am seriously waiting for a solution finn.. :D u hv got me waiting for 2 dys.

Nisarg Thakkar - 7 years, 1 month ago

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@Nisarg Thakkar You seriously want a solution? Fine, I'll give you one now.

Finn Hulse - 7 years, 1 month ago

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@Finn Hulse :D Thanx! If never wanted a solution, I would not have commented on this problem! And if I already had a solution, I would have posted it. But thanx for sharing the solution.

Nisarg Thakkar - 7 years, 1 month ago

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@Nisarg Thakkar HAHAHAHA that is not a solution. That's a guideline. A solution should be brief as well as beautiful. I just made a really bashy solution and encouraged you guys to come up with a cool solution. :D

Finn Hulse - 7 years, 1 month ago

Problems with phrasing: "Segments A E AE and A C AC intersect at points X X and Y Y , respectively." Intersect what? Each other? B F BF ? I know there is a picture, but problems should have unambiguous wordings.

And you should really add a line saying that A B C D ABCD is a rectangle. Although you can see that it looks like a rectangle, I think it'll be helpful for others.

Mursalin Habib - 7 years, 1 month ago

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Sure thing bro. :D

Finn Hulse - 7 years, 1 month ago

Remember that it's an arbitrary rectangle.

Finn Hulse - 7 years, 1 month ago

@Calvin Lin Copying your style of solutions. :D

Finn Hulse - 7 years, 1 month ago

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Not necessarily a great idea as @Nisarg Thakkar points out. You should also provide enough steps to guide at the crux of the problem.

I like the "Treat the whole thing as a square". Can you explain why we can do that without loss of generality?

Calvin Lin Staff - 7 years, 1 month ago

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Oh, yeah. Because it's an arbitrary rectangle, no matter what you do to it it the ratio will remain the same. So for some, it's easier to work with a square.

Finn Hulse - 7 years, 1 month ago

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@Finn Hulse Maybe, but how do you know the ratio isn't different if it's a rectangle with unequal side lengths? While your method is indeed short and the way I would have done it had I gotten it at AMC, AIME, or some other MCQ contest, it is incomplete unless you justify this portion.

Sreejato Bhattacharya - 7 years, 1 month ago

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@Sreejato Bhattacharya I totally agree dude. But I'm leaving the truly beautiful part of this problem to you guys.

Finn Hulse - 7 years, 1 month ago

because the question says 'arbitary'

btw i use geogebra lol

math man - 6 years, 11 months ago

I feel like coordinate bash is overkilling here. It suffices to ratio chase F Y B F , B X B F \frac {FY}{BF},\frac {BX}{BF} using parallels.

Xuming Liang - 6 years, 11 months ago

a classic geometric sol n

So the required answer should be :: 26+9+30 = 65

any confusions please comment

vishwash kumar - 4 years, 7 months ago
Ahmad Saad
Nov 13, 2016

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