Beauty inside a Square

Geometry Level 2

This is a square with side length 4 4 . Semi circles are drawn on each side of the square (as shown in the picture). Find the area of the shaded (black) part.

3 π 3\pi 4 π 3 4\pi - 3 8 π 16 8 \pi - 16 16 2 π 16 - 2\pi

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27 solutions

Alt text Alt text

In First Image: The radius of the semi circles are 2 2 . Then the area of a semi circle is π 2 2 2 = 2 π \frac{\pi*2^2 }{2} = 2\pi . The area of non-red part is 2 2 π = 4 π 2 * 2\pi = 4\pi .

The area of red part = (Area of Square 4 π ) = 16 4 π - 4\pi) = 16 - 4\pi

In Second Image: The area of red part = 2 ( 16 4 π ) = 32 8 π 2*(16 - 4\pi) = 32 - 8\pi

Area of this white part(our desired shaded part)

= ( = ( Area of Square ( 32 8 π ) = 16 32 + 8 π = 8 π 16 - (32 - 8\pi) = 16 - 32 + 8\pi = 8\pi - 16

I also did it in the same way!

Arpit Sah - 7 years, 3 months ago

the area of the shaded part = the area of the semi circle * 4 - the area of square = 2 2 π 4 2 = 8 π 16 2^2 \pi - 4^2 = 8 \pi - 16

Duy Tran - 7 years, 2 months ago

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I think your solution is incorrect but you got the answer right. You missed this:

2(πr^2 ) - s^2

2(π2^2 )- 4^2

8π-16

David Lancaster - 7 years, 2 months ago

2 power 2 +2

Sandra Mounir Philippe - 7 years, 2 months ago

the idea is right anyway

Sandra Mounir Philippe - 7 years, 2 months ago

VERY GOOD YOU ARE CORRECT

Eng-Mostafa Zinedine - 7 years ago

Let's think in a different way that is unusual but it works well to find the answer. 1st eq : (4 shaded+ 4 unshaded= 16) which is the area of the square, 2nd eq ( 2 shaded+ 1 unshaded =2 Pi) , which is the area of the semi circle. by solving the two equation altogether we find that "2 unshaded= 16- 4 Pi". By using the last value of the unshaded in any of the above equation Like 1st eq : (4 shaded+ 4 unshaded = 16 ). you will get the correct answer which is 4 shaded = 8 pi - 16.

Ashraf AbouSalem - 7 years, 2 months ago

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IT IS VERY NEER TO MY ANSWER

Eng-Mostafa Zinedine - 7 years ago

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انا مش عارف اعبر بس انت اكيد هتفهمي انت عربي (انا قلت لو هنفرض الجزء اللي شبه المثلث ده (س والجزء اللي شبه اللوزه (ص والمساحه المطلوبه 4ص هنجيب معادلتين علشان في مجهولين المعادله الاولي نصف مساحه المربع =2س+2ص المعادله التانيه نصف مساحه الدايره=2ص+س ونحلهم مع بعض نجيب ص المطلوب 4ص

Eng-Mostafa Zinedine - 7 years ago

I just thought that answer should be easily divisible by 4 and got it right by chance

Jaskaran Singh - 7 years, 2 months ago

Area of quadrant= 1/4 * pi *2^2= pi..Area of triangle = 1/2 *2 * 2 = 2.... Area of segment = pi - 2... Total area = 8 ( pi - 2) = 8pi-16

Mauricio Jadulos - 7 years, 2 months ago

I remember my Solid Mensuration days.

Jessrey Mark Solijon - 7 years, 2 months ago

me too!!!!

Toufique Imam - 7 years, 2 months ago

yes i have also done like this

vipul madan - 7 years, 2 months ago

nice solution

bnm cnm - 7 years, 2 months ago

I did it in the same way

Chitranshu Vashishth - 7 years, 2 months ago

area of triangle made of half the side of square which is half of the area of one quarter of the square is 1/2 1/2 1/2= 1/8 Area of the quarter of the circle with radius of 1/2 is square side is pi*(1/2)sq /4 =pi16/ so the are of quarter of a circle is minus 1/8 of the square is pi/16 - 1/8 which is (pi-2)/16. multiplying it by 8 for 8 triangles is (pi -2)/2 the only answer which is multiple of (pi-2)/2 is 8pi -16

Ajee Kamath - 7 years, 2 months ago

exactly the same way as above

Balasundar Gunasekar - 7 years, 2 months ago

Very simple solution indeed!

K.K.GARG,india

Krishna Garg - 7 years, 2 months ago

evn i did in the same way..

jo e - 7 years, 2 months ago

me too did it in the same way

Amara Awan - 7 years, 2 months ago

yes, i did it in the same way! :)

Solehuddin Wahid - 7 years, 2 months ago

sme way bt calculated wrongly...

Lava Kumar Marati - 7 years, 2 months ago

I ALSO DID THE SAME WAY!

Nilesh Phalke - 7 years, 2 months ago

got the same answer. but did it in some other way.

Von Uy - 7 years, 2 months ago

i solved it in the same way !!!

Connor Kenway - 7 years, 2 months ago

We can also assume the shaded region being divided into 4. Then area of one strip can be calculated then multiplied by 4. Result will be same i.e. 8pi-16

Ujjawal Krishnam - 7 years, 2 months ago

solid mensuration by kern and bland, 2nd edition, pg. 43, problem no. 15..

Marvin De Luna - 7 years, 2 months ago

THE SHADED AREA EQUAL TO 8 (ΠR^2 m/360 -the corresponding triangle)so is 8 2^2/4 -2*2/2)

aliki patsalidou - 7 years, 1 month ago

I HAVE AVERY DIFFERENCE ANSWER IT IS SO EASY BUT I CANNOT UPLOAD MY PHOTO

Eng-Mostafa Zinedine - 7 years ago

that's the exact same thing I did, but instead of 2 pi+ 2 pi I did 4 pi.

sophia bograd - 7 years ago

Woah !!! 73 73 upvotes. That's a record I believe.

Nishant Sharma - 6 years, 11 months ago
Tunk-Fey Ariawan
Mar 18, 2014

See the picture below.

Flower in Circle Flower in Circle

Area of shaded (red) part is A = [ semicircle ] [ triangle ] = 1 2 π ( 2 ) 2 1 2 4 2 = 2 π 4 \begin{aligned} {\color{#D61F06}A}&=[\text{semicircle}]-[\text{triangle}]\\ &=\frac{1}{2}\pi(2)^2-\frac{1}{2}\cdot4\cdot2\\ &=2\pi-4 \end{aligned} Area of shaded ( black ) part is A = 4 ( 2 π 4 ) = 8 π 16 \begin{aligned} A&=4(2\pi-4)\\ &=\boxed{8\pi-16} \end{aligned}

Is it a triangle?

Archiet Dev - 7 years, 2 months ago

NICE

ramesh thakur - 7 years, 2 months ago

First of all, we add 4 semicircles with r = 2. We see that the petals are counted twice, and the non-petals are counted once.

Then we subtract by the entire square. The petals are now subtracted so we count them only one. And the non-petals are all gone too.

The area = 4 ( 1 2 π × 2 2 ) 4 2 = 8 π 16 = 4(\frac{1}{2} \pi \times 2^{2}) - 4^{2} = \boxed{8\pi - 16} .

This technique's called inclusion-exclusion. Can be used almost every problem when I do these stuffs. You don't have to find the area of specific part like petals and multiply by number of pieces. This one's also able to do some problems that other techniques can't.

Samuraiwarm Tsunayoshi - 7 years, 2 months ago

i like this one the best!! Its an elegant solution.

SAurav TYagi - 7 years, 2 months ago

This one is the best.

Zain Lohani - 7 years, 2 months ago

Exactly the same as u did

lim reon - 7 years, 2 months ago

thats the way i did

Umer Rauf - 7 years, 2 months ago
Chết Chắc
Mar 18, 2014

Solution Solution

As you can see in the picture, we have:

A E = 32 AE = \sqrt{32} A D = 32 2 AD = \frac{\sqrt{32}}{2} S A B C D = 8 {S}_{ABCD} = 8

call O is outer circle that wrap around ABCD

S O = 4 π { S }_{ O }\quad =\quad 4\pi \\

So the remaining area is equal to

S O S A B C D = 4 π 8 {S}_{O} - {S}_{ABCD} = 4\pi \quad -\quad 8\\

The area of haft petal is

4 π 8 4 = π 2 \frac { 4\pi \quad -\quad 8 }{ 4 } \quad =\quad \pi \quad -\quad 2\\

Finally the area 4 petal is: ( π 2 ) 8 = 8 π 16 \left( \pi \quad -\quad 2 \right) \quad *\quad 8\quad =\quad 8\pi \quad -\quad 16\\

Area of two semi circle drawn from two opposite side =one full circle of dia 4=4π Rest of the two same sizes area =4^2-4π=16-4π. So rest of the 4 same sizes Area=2*(16-4π)=32-8π So finally the shaded Area=Area of Square - Area of the rest 4 same sizes Area=16-(32-8π)=8π-16

Laxman Selvam
Mar 18, 2014

the required area is the square's area subtracted from the area of four semicircles. 8pi - 16

Trevor B.
Mar 11, 2014

Start by finding the area of half of one of the black "petals." This can be achieved by taking half of one of the semicircles (a quarter circle) and subtracting out a triangle. The area of a full circle would be 4 π 4\pi so the area of a quarter circle is π . \pi. The triangle is an isosceles right triangle with leg length 2 , 2, so its area is 2. 2. Subtracting the triangle from the quarter circle will give you the area of half of one of the "petals." This is equal to π 2 , \pi-2, so the area of a full petal is 2 π 4. 2\pi-4. There are 4 4 petals. so the total area of the black "flower" is 8 π 16 \boxed{8\pi-16}

Well explained.

Aji Karunakaran - 7 years, 2 months ago

this was my solution :)

Reginald Micu - 7 years, 2 months ago
Harikesh Tripathi
Apr 23, 2014

The entire area inside the square is covered by four semicircles( including the area which is shaded and covered twice) , four semicircles are equal to two circle. radius of the circle is 2 ( side of square is 4). Are of two circles with radius to 2 is 8* PIE. As the four semicircles cover the whole area of the square + the area of intersection. So the area of intersection i.e shaded part is 8 PIE - (area of square) = 8 PIE -16.

Imam Edogawa
Apr 16, 2014

just take a litle part of this cicle. then power 8. so the answer is 8phi-16

Prakash Padhy
Mar 31, 2014

Let the area of the shaded portion =4x, (4 eqal segments), Area of the semi circle each is = s, and the area other than the shaded portions in the semi circle = y, As we can know from the picture, 2x=s-y, Hence 4x= 2s-2y, ...(a) Also 4x=(area of the squre)-4y, = 16-4y, ...(b), Hence (a)=(b), i.e. 2s-2y = 4^2 -4y, 2(2#)-2y = 16-4y, Hence on solving we can get y = 8-2#, Hence putting on (a) we get 4x = 2(2#) - 2(8 - 2#), = (8# - 16),
= The required answer.

Take a part of shaded portion as Y. Take a part of unshaded portion as X. Taking whole square into account, 4X + 4Y=16 (Area of square=16) X+Y=4.....Take it as equation1.. Taking a semi circle into account, 2Y+X=2Pi (Area of semicircle=2Pi) Take it as equation2.. Solve 1 and 2... Y=2Pi-4, 4Y=8Pi-16......

Anandhu Raj
Mar 28, 2014

Area of shaded region = (sum of areas of 4 semicircles) - area of square = 9.1327 \approx 8 π \pi -16

Aditya Vikram
Mar 27, 2014

16-{8(4-pi)}=ans

Onkar Habbu
Mar 27, 2014

Calculate area of 1 semi-circle and multiply it by 4. Now you get the area of square + area of shaded region. Therefore, subtract area of square from it to get area of shaded region.

Ajith Gade
Mar 25, 2014

total area of square =16; area occupied by two semi circles =4 pi; two parts of unshaded area =16-4 pi;total unshaded area =2(16-4 pi); shaded area =16-unshaded area;16-2(16-4 pi);8*pi-16

Subhadip Biswas
Mar 23, 2014

4 half circle area 8pi area of black shaded area =area of squre-8pi=16-8pi

8pi -16

area of 4 semicircles -area of square

Amith Pottekkad
Mar 23, 2014

There are 4 semi circles,i.e 2 circles.The area of one semi circle=pi 2 2/2,,,=2pi. If we consider,2 semi circle,which lies opposite,the total area of the 2 semi circle is 4pi.So the area of the rest part =16-4π.And there is 2 semi circle and remaining part.The area of the remaining =16-4π.So the area of the shaded part=16-(the area of square)32-8π 16-(32-8π),,=16-32+8π= 8π-16

Saad Leghari
Mar 22, 2014

Simply: First calculate the area of whole square i.e. 4 x 4 = 16 Then calculate the area of one whole circle (note: there are two full circles inside) i.e. 4π Now subtracting 4π from 16 gives = 16-4π 16-4π/2 = 8-2π which is not-shaded-upper-and-lower part each inside the square Area of a semi-circle i.e. π4/2 = 2π Now subtract 8-2π from 2π i.e. 4π-8 4π-8/2 = 2π-4 (each of the above shaded parts) Hence, 4* (2π-4) = 8π-16.

Ankur Jyoti Das
Mar 20, 2014

area of the small triangle=(1/2) 2 2=2 area of one fourth of circle=1/4(pi 4)=pi area of one such shaded portion=2 (pi-2) reqd area=4 2 (pi-2) =8pi -16

V Abhishek
Mar 19, 2014

Sides of square=4cm radius of semicircle=sides of square/2 = 2 cm

since there a four semicircles area of shaded region = area of 4 semicircle-area of square =4 πr^2 - 4 4 -------- 2 =2 π 4 - 16 =8π-16

Melvin Mosq
Mar 19, 2014

area of square=4*4=16 now a=one of the intersected small region like banana b=one of the other region like Eiffel tower in paris. then area of semi circle=2 pi we can get 2 equations 4a+4b=16 and 2a+b=2 pi then solve ,gives a=(8 pi-16)/4 required area=4 a=8 pi-16 hence the answer.

Shivam Sharma
Mar 19, 2014

there are 4 black parts and 4 white parts. now,

1 black part = x

1 white part = y

now, area of the square = 4 * (x) + 4 * (y) .

16 = 4x + 4y........eq.(1)

also,

area of the semi circle = 2 * (x) + y

0.5 * (pi) * 4 = 2 * (x) + y

2(pi) = 2x +y..........eq. (2)

solving these 2 equations , we get x =2(pi) - 4

hence, area of black part is 4 * (x) = 8 (pi) - 16

David Lancaster
Mar 18, 2014

Taking a quarter of the big square we now have a 2 by 2 square. The shaded area is 8 segments of a circle. To get the area of a segment of a circle is area of a sector minus the area of the triangle.

Area of a sector is:

(r^2/2)∅ , where ∅ is equal to π/2 in radian which gives

(r^2/2)*(π/2)=(πr^2)/4

Area of a triangle is:

bh/2 = (2*2)/2 = 2

Area of a segment is:

(πr^2/4)-2

Multiplying this into 8 we get:

2πr^2-16

Putting 2 as r we get:

8π-16

Bhavesh Bhagde
Mar 18, 2014

1st vve find the area of semicircle vvith radius is 2, area of shaded region=sum of area of 4 semicircle - area of square

Alif Hasnat Dip
Mar 18, 2014

The little dark (1 of total 4) area is A=2 (1/2) r^2 (theta(radian) - sin (theta)) , theta = 90, r=2 So total dark area will be 4 A

Venture Hi
Mar 9, 2014

Reduce the problem into the smaller square of 2x2. Integrate y=4 - x^2 from 0 to 2 ( formula for the circle is x^2+y^2=4 for the semi circle inside the 2x2 square) The result is pi. This area minus the equilateral triangle of 2x2 gives you pi-2. Multiply by 2 to get the shaded area of the 2x2 square = 2pi-4. Since there are 4 of these little 2x2 squares, multiply the total by 4. = 8pi-16.

the circle is y=sqrt(4-x^2). Sorry foe the typo.

Venture HI - 7 years, 3 months ago

8π−16

Fahima Rahman - 7 years, 2 months ago

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