What is the area of the region enclosed by the graph of x 2 + y 2 = ∣ x ∣ + ∣ y ∣ ?
Notation : ∣ ⋅ ∣ denotes the absolute value function .
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Check out AMC 2016 12B problem 18. exact same problem :)
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Or 2016 AMC 10B problem 21. I got it from there.
Wow, that's such a pretty image.
Wow that was great
The polar equation of the portion of the bounding curve in the first quadrant is r = cos θ + sin θ 0 ≤ θ ≤ 2 1 π and so the area enclosed (by symmetry) is 4 × 2 1 ∫ 0 2 1 π ( cos θ + sin θ ) 2 d θ = 2 ∫ 0 2 1 π ( 1 + sin 2 θ ) d θ = π + 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..
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With x = r cos θ and y = r sin θ the equation becomes r 2 = ∣ r cos θ ∣ + ∣ r sin θ ∣ and so r = ∣ cos θ ∣ + ∣ sin θ ∣
the edge of the square = the diameter of the half circles= √
(
1
2
+
1
2
)
= √2
total area = area of square + area of 4 half circles = ( √ 2 ) 2 + ( 4 × 2 4 × π × ( √ 2 ) 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
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.
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First we consider the case where x , y ≥ 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 − x + 4 1 + y 2 − y + 4 1 = 2 1
( x − 2 1 ) 2 + ( y − 2 1 ) 2 = 2 1 = ( 2 2 ) 2
So, we know the circle has center ( 2 1 , 2 1 ) and radius 2 2 . We also know that it crosses the axes at ( 1 , 0 ) and ( 0 , 1 ) . By symmetry, we can tell that there are like circles in all four quadrants for each of the cases of x and 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 . The semicircles each have area 2 1 π ( 2 2 ) 2 = 4 1 π . Therefore, the combined area is π + 2 .