Slightly inflated square

Geometry Level 4

The figure above consists of 4 identical intersecting quadrants inside a square of side 4. The area of the shaded region can be expressed as a b π + c d e \frac{a}{b}\pi+c-d\sqrt{e} for positive integers a , b , c , d a,b,c,d with e e with a a and b b are relatively coprime, and e e is square free.

Evaluate the value of a + b + c + d + e a+b+c+d+e .


The answer is 54.

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7 solutions

Keil Cerbito
Jul 16, 2015

A A F E C D A = π 4 ( 4 2 ) = 4 π A_{AFECDA} = \frac{\pi}{4}(4^{2}) = 4\pi

A A E G D A = π 6 ( 4 2 ) = 8 π 3 A_{AEGDA} = \frac{\pi}{6}(4^{2}) = \frac{8\pi}{3}

A A F E A = π 6 ( 4 2 ) ( 4 2 ) sin ( 60 ) 2 = 8 π 3 4 3 A_{AFEA} = \frac{\pi}{6}(4^{2}) - \frac{(4^{2})\sin{(60)}}{2} = \frac{8\pi}{3} - 4\sqrt{3}

A D G E C D = A A F E C D A A A E G D A A A F E A A_{DGECD} = A_{AFECDA} - A_{AEGDA} - A_{\overline{AFEA}}

A D G E C D = 4 π 8 π 3 ( 8 π 3 4 3 ) = 4 π 3 + 4 3 A_{DGECD} = 4\pi - \frac{8\pi}{3} - (\frac{8\pi}{3} - 4\sqrt{3}) = \frac{-4\pi}{3} + 4\sqrt{3}

A E F G H E = A s q u a r e 4 A D G E C D A_{EFGHE} = A_{square} - 4A_{DGECD}

A E F G H E = 4 2 4 ( 4 π 3 + 4 3 ) A_{EFGHE} = 4^{2} - 4( \frac{-4\pi}{3} + 4\sqrt{3})

A E F G H E = 16 π 3 + 16 16 3 A_{EFGHE} = \frac{16\pi}{3} + 16 - 16\sqrt{3}

t h e r e f o r e : therefore:

a = 16 ; b = 3 ; c = 16 ; d = 16 ; e = 3 a=16 ; b=3; c=16; d=16; e=3

a + b + c + d + e = 16 + 3 + 16 + 16 + 3 = 54 a+b+c+d+e = 16+3+16+16+3 = \boxed{54}

@Keil Cerbito on the second line it should be A subscript(AFED)

Joshua Daniel - 5 years, 11 months ago

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thanks for correcting my solution @Joshua Daniel . Actually I had some minor mistakes there, the area that I am talking about on the 2nd line is the area of Sector AEGD.

Keil Cerbito - 5 years, 11 months ago

area of AEGDA = pi/3 (4^2) but you wrote pi/6(4^2)

Ravi Nelluri - 5 years, 11 months ago

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Good day Sir @Ravi Nelluri . If you recall the formula for getting the Area of a Sector, it should be A = 1 2 θ r 2 A = \frac{1}{2}\theta r^{2} wherein θ = 60 ° = π 3 \theta = 60° = \frac{\pi}{3} and r = 4 r = 4 . Substitute to the equation, we will get A = 1 2 ( π 3 ) ( 4 ) 2 = π 6 ( 4 ) 2 = 8 π 3 A = \frac{1}{2}(\frac{\pi}{3})(4)^{2} = \frac{\pi}{6}(4)^{2} = \frac{8\pi}{3}

Keil Cerbito - 5 years, 11 months ago

We can calculate the area given by the intersection of the four quadrants as the sum of : 4 A r e a D F C + 2 A r e a D B A r e a A B C D = A r e a E F G H 4Area_{DFC}+2Area_{DB}-Area_{ABCD}=Area_{EFGH}

Consider the notation as the area enclosed by the lines (No matter if they are curved) that connect the given points corresponding the image above. We call the side of the square r r . Then:

A r e a D B = w π r 2 4 × 2 w = r 2 A r e a D B = π r 2 2 r 2 Area_{DB}=w\Rightarrow \dfrac{\pi r^2}{4}\times 2-w= r^2\Rightarrow Area_{DB}=\dfrac{\pi r^2}{2}-r^2

In order to calculate the area of D F C DFC we note that triangle Δ A B F \Delta_{ABF} is equilateral . So its area is :

r 3 2 × 1 2 × r = r 2 3 4 \dfrac{r \sqrt{3}}{2}\times\dfrac{1}{2}\times r=\dfrac{r^2 \sqrt{3}}{4}

The sum of the area of two circular sectors of 6 0 60^{\circ} is: π r 2 3 \dfrac{\pi r^2}{3}

The sum of the area of two quadrants is: π r 2 2 \dfrac{\pi r^2}{2}

Then the area of D F C DFC is:

r 2 ( π r 2 2 ( π r 2 3 r 2 3 4 ) ) = r 2 π r 2 6 r 2 3 4 r^2-\left (\dfrac{\pi r^2}{2}-\left (\dfrac{\pi r^2}{3}-\dfrac{r^2 \sqrt{3}}{4}\right )\right )=r^2-\dfrac{\pi r^2}{6}-\dfrac{r^2 \sqrt{3}}{4}

Finally, the generalized form of the area in common of the 4 4 quadrants is:

A r e a E F G H = x x = 2 ( π r 2 2 r 2 ) + 4 ( r 2 π r 2 6 r 2 3 4 ) r 2 Area_{EFGH}= x\Rightarrow x= 2\left (\dfrac{\pi r^2}{2}-r^2\right )+4\left (r^2-\dfrac{\pi r^2}{6}-\dfrac{r^2 \sqrt{3}}{4}\right)-r^2

= π r 2 3 + r 2 r 2 3 =\boxed{\dfrac{\pi r^2}{3}+r^2-r^2\sqrt{3}}

Substituting r, we get:

16 π 3 + 16 16 3 \dfrac{16 \pi}{3}+16-16\sqrt{3}

So the answer is:

16 + 3 + 16 + 16 + 3 = 54 16+3+16+16+3=\boxed{\boxed{54}}

On 6th May 2016 I had answered almost the same equation but square side of 1 instead of 4 as it is here "Blue area by Alan Enrique Ontiveros ".
The above is the copy of that solution. Since the square here has side 4, the required area is:-
4 2 π + 3 3 3 3 = 16 3 + 16 16 3 = a b π + c d 3 . 4^2*\dfrac{\pi+3-3\sqrt3} 3=\frac{16 } 3+16-16\sqrt3=\frac{a } b*\pi+c-d\sqrt3.
So a+b+c+d+e=16+3+16+16+3=54.
This is the fifth time almost the same equation has appeared. Bob Kadylo below too has same thing to say.



Bob Kadylo
Feb 17, 2016

At the end of January 2016, I did a problem called "Area Of Overlapping Leaves" - Geometry Level 3 - which currently awards 60 points. Lots of detailed beautiful colored solutions I learned from - that were different from the way I did it - but I love that. Anyway, here is the result: a 2 3 ( π + 3 3 3 ) \frac{a^{2}}{3}(\pi+3-3\sqrt{3})

I just applied that general result to this question using "a=4" and multiplied 16 3 \frac{16}{3} into the brackets.

John Rowland
Jul 17, 2015

Yes 54: Using the notation in Keil Cerito's diagram: Take A as the origin, G has co-ordinates (√12,2). Then 4 x (Int - 2(√12-2)) where Int is the area under the curve x^2+y^2 = 4^2 from 2 to √12! It works! (Also interesting that the value of Int is 1/3 of the area of the quarter circle.)

Moderator note:

Your solution is quite hard to follow. Adding additional details would make it easier to read.

How can one determine the coordinates of G? (e.g. did you use the fact about equilateral triangles? did you use a graphing software?) How can one determine the area of Int? (e.g. did you approximate the area? did you use a graphing software?)

Is there any reason why Int is 1/3 the area of the quarter circle, or is that a happy coincidence?

Mikael Marcondes
Jul 17, 2015

Set the origin of a system of coordinates at the center of the square. Using the circumferences' equations centered at the vertices V 1 = ( 2 , 2 ) ; V 2 = ( 2 , 2 ) ; V 3 = ( 2 , 2 ) and V 4 = ( 2 , 2 ) V_1=(2, 2); V_2=(2, -2); V_3=(-2, -2) \text{ and } V_4=(-2, 2) one may find the intersection between the circumferences of V 1 V_1 and V 2 V_2 and integrate the function of V 2 V_2 over the interval [ 2 12 , 0 ] [2-\sqrt{12},0] (i. e. from the intersection to the origin). This is a quarter of the shaded region, thus we multiply the value achieved by four and hence the answer.

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