Area problem

Geometry Level 3

A B C D ABCD is a square with points E E and F F lying on sides C D CD and A D , AD, respectively. If the purple area is [ B H G I ] = 120 , [BHGI]=120, what is the sum of the pink areas [ A H F ] + [ F G E D ] + [ I C E ] ? [AHF]+[FGED]+[ICE]?


The answer is 120.

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8 solutions

Dan Ley
Apr 17, 2017

In each case, the area shaded is half of the area of the square. Since the red regions added are the same each time, the purple and pink areas must both have the same area of 120 \boxed{120} .

Simple and effective. Like all the best mathematics.

Malcolm Rich - 4 years, 1 month ago

ok its half square....but from where does this half square came....i mean how do we know that its half square

A Former Brilliant Member - 4 years, 1 month ago

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It's half the area of the square because it consists of a triangle with base = height = square side length:)

Dan Ley - 4 years, 1 month ago

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Oh, man. DUH..

Alex Maione - 4 years, 1 month ago

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@Alex Maione shame....man...stop solving maths on ur imagination

A Former Brilliant Member - 4 years, 1 month ago

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@A Former Brilliant Member The sides of a square are equal....so the base and height becomes equal...basics

Ben John Toms - 3 years, 6 months ago

its not given that base,height and square side length are equal......:)

A Former Brilliant Member - 4 years, 1 month ago

E and F are lying on the sides they arent the midpoints then how can you state that the area is half

Khushaal Nandwani - 4 years, 1 month ago

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Because regardless of whether it's the midpoint, the area of the triangle is 0.5 b h. The base and height are each equal to the sides of the square. Therefore the triangle's area is 0.5*s^2. And the area of the square is simply s^2.

Danielle Manning - 4 years, 1 month ago

Lol am sorry i just realised my mistake

Khushaal Nandwani - 4 years, 1 month ago

Not sure this is mentioned anywhere, so I'll say it: The name of this type of problem is a Carpet Theorem problem, and there are many other cool ones like it.

Isaac Browne - 3 years, 9 months ago
Chew-Seong Cheong
Apr 10, 2017

Let the area of the square [ A B C D ] = A s q u a r e [ABCD] = A_{square} , the blue area be A p u r p l e A_{\color{#69047E}purple} and the sum of the pink areas be A p i n k A_{\color{#D61F06}pink} . Note that:

{ A p u r p l e + [ F G H ] + [ B C I ] = [ B C F ] = 1 2 A s q u a r e A p i n k + [ F G H ] + [ B C I ] = [ A D E ] + [ B C E ] = 1 2 A s q u a r e A p i n k = A p u r p l e = 120 \begin{cases} A_{\color{#69047E}purple} + [FGH] + [BCI] & = [BCF] & = \frac 12 A_{square} \\ A_{\color{#D61F06}pink} + [FGH] + [BCI] & = [ADE] + [BCE] & = \frac 12 A_{square} \end{cases} \implies A_{\color{#D61F06}pink} = A_{\color{#69047E}purple} = \boxed{120}

The title of the problem should be -"The Answer Lies Within"

Aniruddha Bagchi - 4 years, 2 months ago

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Yes, you are right.

Chew-Seong Cheong - 4 years, 2 months ago

Your explanation is valid only if the triangles are equal. If not, despite the answer be the same, your explanation doesn't apply.

João Lima - 4 years, 1 month ago

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I think you mean if [ B C F ] = [ A D E ] + [ B C E ] [BCF] = [ADE]+[BCE] . They are equal. As we have [ A D E ] = 1 2 × D C × B C [ADE] = \frac 12 \times DC \times BC and [ A D E ] + [ B C E ] = 1 2 × B C × D E + 1 2 × B C × E C = 1 2 × B C ( D E + E C ) = 1 2 × B C × D C = [ A D E ] [ADE]+[BCE] = \frac 12 \times BC \times DE + \frac 12 \times BC \times EC = \frac 12 \times BC (DE+EC) = \frac 12 \times BC \times DC = [ADE] .

Chew-Seong Cheong - 4 years, 1 month ago

It is correct for any partition.

Ion Filipski - 2 years, 10 months ago

Good work!

Ion Filipski - 2 years, 10 months ago

Clear explanation

Manvitha Reddy - 1 year, 8 months ago
Bryan Hung
Apr 16, 2017

This is a slight "cheating" answer, but since it is implied that a unique answer exists, the positions of points F on AD and E on DC shouldn't matter.

Slide point F to A and slide point E to C. Quadrilateral BHGI is now a right triangle, and half of the square. The pink areas form the other half of the square. So, under the assumption that there is a unique answer, the areas are the same.

My first thoughts also. Often times taking a problem to its limit shows a simple solution.

A Former Brilliant Member - 4 years, 1 month ago

That's exactly what I did! Nice.

John Smith - 4 years, 1 month ago

Incorrect If f and e moved to point d the area would be 0, it happens to work in this scenario but its not a proof.

David Turner - 4 years, 1 month ago

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If F and E are moved to D, then the purple area will become zero. We know that it is 120, so this case will not work.

Pranshu Gaba - 4 years, 1 month ago

I'm sorry? For any math question, if I can determine that a variable's change does not affect the problem, and that changing this variable simplifies the problem greatly, I have every right to use this fact to my advantage.

The beginning of my answers says "This is a slight 'cheating answer' ", because it's obviously not a full proof. It's a shortcut to the answer.

Bryan Hung - 4 years, 1 month ago

Not incorrect. The problem did not ask for proof. It asked for a numerical answer. One can logically see that the statement of the problem implies that the simple case, where F is A and E is C, must hold.

george palen - 2 years, 12 months ago

moving F and E to D makes the square expand to infinity, so it is not a problem

Jeremy Ho - 4 years, 1 month ago

Starting from Δ A F H \Delta AFH and going in a clockwise rotation, designate the areas of the distinct regions as a , b , c , d , e , f , g , h a,b,c,d,e,f,g,h . (In this designation, then, the areas of the red regions will be a , e , g a, e, g , the area of the blue region will be c c and the areas of the white regions b , d , f , h b, d, f, h .)

Now as A F + F D |AF| + |FD| is equal to the side length of the square, we know that Δ A B F + Δ F C D |\Delta ABF| + |\Delta FCD| will equal half the area of the square. But so too will Δ A B E |\Delta ABE| . Therefore

Δ A B F + Δ F C D = Δ A B E a + b + e + f + g = b + c + f a + e + g = c |\Delta ABF| + |\Delta FCD| = |\Delta ABE| \Longrightarrow a + b + e + f + g = b + c + f \Longrightarrow a + e + g = c .

But this just means that the sum of the areas of the 3 red regions is equal to that of the blue region, and so the desired sum is 120 \boxed{120} .

How u say that ar(ABF)+ar(FCD) will half of the ar. Of square

Gautam Kumar - 4 years, 1 month ago

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The area of Δ A B F \Delta ABF is equal to 1 2 × A F × A B \dfrac{1}{2} \times |AF| \times |AB| .

The area of Δ F C D \Delta FCD is equal to 1 2 × F D × D C \dfrac{1}{2} \times |FD| \times |DC| .

Since D C = A B |DC| = |AB| , the sum of the areas of the two triangles is

1 2 ( A F + F D ) × A B \dfrac{1}{2}(|AF| + |FD|) \times |AB| .

But A F + F D = A D |AF| + |FD| = |AD| , so the sum of the areas is

1 2 × A D × A B \dfrac{1}{2} \times |AD| \times |AB| , which is half the area of the square.

Brian Charlesworth - 4 years, 1 month ago
Mesay Solomon
Apr 22, 2017

Area of the non-pink region equals |AEB|+|CFB| - 120 which equals |ABCD| - 120 (since |AEB| + |CFB| = |ABCD|).

Area of the pink region = |ABCD| - area of non-pink region = |ABCD| - (|ABCD| - 120) = 120

Angel Krastev
Jun 18, 2017

The answer is 120.
Let the areas of each part are as shown on the picture: We can write and solve two equations for PA(PinkArea) = UL + DL + DR
(1) [ABF]+[FCD]=[BCF] for any position of F on AD
(2) [DAE]+[EBC]=[ABE] for any position of E on DC


           ML+120+R = UL+U+DL+MD+DR = PA+U+MD   
           U+120+MD = UL+ML+DL+R+DR = PA+R+ML

        ML+120+R = PA+U+MD
     +  U+120+MD = PA+R+ML 
  ----------------------------------      
   ML+MD+U+R+240 = ML+MD+U+R+2PA -> 2PA=240 -> PA=120
Martin Deady
Apr 30, 2021

I simply cheated with this one. Because I am lazy I simply made points F and G coincident with D and C the problem is then trivial. An engineers approach! ,

Reframing a problem.

Martin Deady - 1 month, 1 week ago
Sophia Cristina
Aug 13, 2018

[ A H B ] = [ F G E D ] + [ F H G ] [AHB] = [FGED] + [FHG]

Half Square:

= [ A H B ] + [ F H G ] + 120 = [AHB] + [FHG] + 120

= [ A H B ] + [ F H G ] + [ A H F ] + [ I C E ] + [ F G E D ] = [AHB] + [FHG] + [AHF] + [ICE] + [FGED]

Then:

[ A H B ] + [ F H G ] + 120 = [ A H B ] + [ F H G ] + [ A H F ] + [ I C E ] + [ F G E D ] [AHB] + [FHG] + 120 = [AHB] + [FHG] + [AHF] + [ICE] + [FGED]

120 = [ A H F ] + [ F G E D ] + [ I C E ] 120 = [AHF] + [FGED] + [ICE]

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