What Does It Look Like?

Geometry Level 3

What is the area of the region enclosed by the graph of x 2 + y 2 = x + y x^2+y^2=|x|+|y| ?

Notation : | \cdot | denotes the absolute value function .

2 π + 2 2\pi+\sqrt{2} π + 2 \pi+2 π + 2 2 \pi+2\sqrt{2} 4 4

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4 solutions

Aaron Tsai
Aug 11, 2016

First we consider the case where x , y 0 x,y\geq 0 to eliminate the absolute value signs. We can manipulate equation to rewrite it in the standard form of a circle :

x 2 x + y 2 y = 0 x^2\textcolor{#3D99F6}{-x}+y^2\textcolor{#3D99F6}{-y}=0

x 2 x + 1 4 + y 2 y + 1 4 = 1 2 x^2-x \textcolor{#3D99F6}{+\dfrac{1}{4}}+y^2-y \textcolor{#3D99F6}{+\dfrac{1}{4}}=\textcolor{#3D99F6}{\dfrac{1}{2}}

( x 1 2 ) 2 + ( y 1 2 ) 2 = 1 2 = ( 2 2 ) 2 \left(x-\dfrac{1}{2}\right) ^2 + \left(y-\dfrac{1}{2}\right) ^2 = \dfrac{1}{2} = \left(\dfrac{\sqrt{2}}{2}\right) ^2

So, we know the circle has center ( 1 2 , 1 2 ) \left(\dfrac{1}{2} , \dfrac{1}{2}\right) and radius 2 2 \dfrac{\sqrt{2}}{2} . We also know that it crosses the axes at ( 1 , 0 ) (1,0) and ( 0 , 1 ) (0,1) . By symmetry, we can tell that there are like circles in all four quadrants for each of the cases of x x and y y being positive/negative. Thus, the graph of the equation is as shown:

Note that the region is made up of 4 semicircles lying on sides of a square with side length 2 \sqrt{2} . The semicircles each have area 1 2 π ( 2 2 ) 2 = 1 4 π \dfrac{1}{2}\pi \left(\dfrac{\sqrt{2}}{2}\right) ^2=\dfrac{1}{4} \pi . Therefore, the combined area is π + 2 \boxed{\pi+2} .

Check out AMC 2016 12B problem 18. exact same problem :)

Raymond Park - 4 years, 10 months ago

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Or 2016 AMC 10B problem 21. I got it from there.

Aaron Tsai - 4 years, 10 months ago

Wow, that's such a pretty image.

Calvin Lin Staff - 4 years, 10 months ago

Wow that was great

space sizzlers - 4 years, 9 months ago
Mark Hennings
Aug 19, 2016

The polar equation of the portion of the bounding curve in the first quadrant is r = cos θ + sin θ 0 θ 1 2 π r \; = \; \cos\theta + \sin\theta \qquad \qquad 0 \le \theta \le \tfrac12\pi and so the area enclosed (by symmetry) is 4 × 1 2 0 1 2 π ( cos θ + sin θ ) 2 d θ = 2 0 1 2 π ( 1 + sin 2 θ ) d θ = π + 2 4 \times \tfrac12\int_0^{\frac12\pi}(\cos\theta + \sin\theta)^2\,d\theta \; = \; 2\int_0^{\frac12\pi} (1 + \sin2\theta)\,d\theta \; = \; \pi+2

Wow..such a cool solution..i think the crux move was to find out the polar relationship...can you please show me how to find that? I'm having problem with that..

Istiak Reza - 4 years, 10 months ago

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With x = r cos θ x=r\cos\theta and y = r sin θ y=r\sin\theta the equation becomes r 2 = r cos θ + r sin θ r^2 = |r\cos\theta| + |r\sin\theta| and so r = cos θ + sin θ r = |\cos\theta|+|\sin\theta|

Mark Hennings - 4 years, 10 months ago

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Got it...thanks..

Istiak Reza - 4 years, 9 months ago
Mohtasim Nakib
Aug 28, 2016

the edge of the square = the diameter of the half circles= √ ( 1 2 + 1 2 ) (1^2 + 1^2) = √2

total area = area of square + area of 4 half circles = ( 2 ) 2 + ( 4 × π × ( 2 ) 2 4 × 2 (√2)^2 + (\frac{4×π×(√2)^2}{4×2} ) = π + 2 π + 2

N.B. the sides of the square is the diameter of the half circles because it contains the centers of the circles: x 2 + y 2 ± x ± y = 0 x^2 + y^2 ± x ± y = 0

As shown by Mohtasim Nakib, by graphing the function and realizing that it is a square and four semicircles, I got the area.

The idea is to figure out what the shape of the graph is, without actually graphing it. This problem is originally from a math competition, so you couldn't graph using a computer, and graphing it by hand would take too much time.

Aaron Tsai - 4 years, 9 months ago

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Agreed. Thanks.

Niranjan Khanderia - 4 years, 9 months ago

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