Don't overthink

Calculus Level 3

R d A \large \iint_{R} \, dA

Find the value of the above integral over a region R R enclosed by

x y + x + y = 2017. |x-y| + |x+y| = 2017.


The answer is 4068289.

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2 solutions

Efren Medallo
Apr 25, 2017

This problem basically asks for the area bounded by the graph of

x + y + x y = 2017 |x+y| + |x-y| = 2017

which, graphically, is

a square centered at the origin with side length 2017 2017 . And thus, we can easily get its area, and that would be 201 7 2 = 4068289 2017^2 = \boxed{4068289}

We can actually generalize the graph of a square centered at the origin for any side length k k . That would be x + y + x y = k |x+y| + |x-y| = k And the "unit square" will have the equation with k = 1 k=1 .

Vincent Moroney
Jul 11, 2018

Since the obvious graphical solution was posted, here's a computational solution. Let's first start with the inequality x y + x + y 2017. |x-y| + |x+y| \leq 2017. If x > y x>y then we have x 2017 2 x \leq \frac{2017}{2} . If x < y x<y such that x + y > 0 x+y>0 , then we get y 2017 2 y \leq \frac{2017}{2} . If x < y x< y such that x + y < 0 x+y<0 , then we get x 2017 2 x \geq \frac{-2017}{2} . If y < x y<x such that x + y < 0 x+y<0 , then we get y 2017 2 y \geq \frac{-2017}{2} . Using this we can construct the integral A = 2017 2 2017 2 2017 2 2017 2 d x d y A = \int_{-\frac{2017}{2}}^{\frac{2017}{2}}\int_{-\frac{2017}{2}}^{\frac{2017}{2}} \,dx\,dy A = 4 ( 2017 2 ) 2 = 201 7 2 = 4068289 . A = 4\Big(\frac{2017}{2}\Big)^2 = 2017^2 = \boxed{4068289}.

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