Perfect fit

Geometry Level 2

Consider the image above. Let R R and B B denote the area of the red section and blue section respectively. If the area of the square is 1024 cm 2 1024 \text{ cm}^2 , find B R \frac B R . Give your answer to 3 decimal places.

Note - The image is up to scale.


The answer is 0.273.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

5 solutions

Discussions for this problem are now closed

Steven Zheng
Dec 28, 2014

The size of the square is not important, so I will scale the square with a side length of 2.

Observe that 4 times of the red area is the circle, and 4 times of the blue area is the square minus the circle.

Thus the ratio is A s q u a r e A c i r c l e A c i r c l e \frac{{A}_{square}-{A}_{circle}}{{A}_{circle}} for my values, we get 4 π π . \frac{4-\pi}{\pi}.

the area of fig=4B+4R=1024 B+R=256 R=256-B AREA OF FIG=X^2=1026 X=32 r=16 R=.25 22/7 16*16=201.14 B=256-R=256-201.14=54.85 B/R=54.85/201.14=0.272

Mina Maher - 6 years, 4 months ago

guys i explain to me how you got the answer

Brilliant Smart - 6 years, 5 months ago

I've added a bit more detail to the solution to make it much clearer. Does this help?

Calvin Lin Staff - 6 years, 5 months ago
Bob Melms
Jan 3, 2015

The actual area of the square doesn't matter because it is a ratio problem. You'll see that r cancels out:

Mahrez Boufares
Jan 2, 2015

diameterof the cercle √1024 = 32 So r = 16

area of the circle =(16)^2 pi = 256 pi

R= (256/4) pi = 64 pi = 200.96

B = (1024 - 803.84)/4 =(220.16 / 4 ) = 55.04

B/R = 0.2738854

Jack Rawlin
Dec 28, 2014

First you need to find the area of the circle to do this you need the radius. To find the radius you square root the area of the square to get the length of the square

1024 = 32 \sqrt{1024} = 32

Since the length of the square is the same as the diameter of the circle you just need to halve it to find the radius.

32 2 = 16 \frac {32}{2} = 16

So r = 16 r = 16

Next we need to find the area of the sector in the circle (the red bit). To do this we first need to find the total area of the circle.

a = π r 2 a = \pi r^2

In terms of π \pi this gives us a = 256 π c m 2 a = 256\pi cm^2

Since the image is to scale the red sector has an angle of 9 0 90^\circ . So to find the sector we use this formula

a s e c t o r = a A n g l e 360 a_{sector} = \frac {a\cdot Angle^\circ}{360}

So the area of the sector is

a s e c t o r = 256 π 9 0 360 = 64 π c m 2 a_{sector} = \frac {256\pi\cdot90^\circ}{360} = 64\pi cm^2

R = 64 π c m 2 R = 64\pi cm^2

We now have the red area but what about the blue?

The blue area is (since the image is to scale) about one quarter of the area left when you subtract the area of the circle from the area of the square so

B = 1024 c m 2 256 π c m 2 4 219.752 c m 2 4 54.938 c m 2 B = \frac {1024cm^2 - 256\pi cm^2}{4} \approx \frac {219.752cm^2}{4} \approx 54.938cm^2

So

B R 54.938 c m 2 256 π c m 2 0.273 \frac {B}{R} \approx \frac{54.938cm^2}{256\pi cm^2} \approx 0.273

Anoj Ahilan
Jan 7, 2015

To calculate this ratio, the area of the square is irrelevant. Let “X” be the radios of the circle. So width and height of the square is 2 x X. Area of the circle = πX^2 Area of the square = (2X)2 = 4X2

Area of R = πX^2 / 4

Area of B = (4X2 / 4) - πX^2 / 4 = (4X2 - πX^2) / 4

B / R = (4X2 - πX^2) / (4 (πX^2 / 4)) = (4X2 - πX^2) / πX^2 = (4 – π) / π

Assuming π = 22/ 7,

B / R = (4 * 7) / 22 – 1 = 28 / 22 – 1 = 6 / 22

Answer: B / R = 3 / 11

It didn't mention that we assume π = 22 7 \pi = \frac {22}{7} so we shouldn't assume it, but good job nonetheless!

Pi Han Goh - 6 years, 2 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...