Integral over a Really Weird Region

Calculus Level 5

If

E 1 x y d x d y = A \iint_{E} \frac{1}{xy} dxdy = A

where E E is the region bounded by the four curves:

  1. x 2 + y 2 = x \ x^{2}+y^{2} = x
  2. x 2 + y 2 = 2 x \ x^{2}+y^{2} = 2x
  3. x 2 + y 2 = y \ x^{2}+y^{2} = y
  4. x 2 + y 2 = 2 y \ x^{2}+y^{2} = 2y

Find 1000 A \lfloor{1000A}\rfloor .

Notation: \lfloor \cdot \rfloor denotes the floor function .


The answer is 480.

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2 solutions

This picture shows roughly the area we want to integrate in(I am very very poor at drawing)

We have to use a variable change to solve the problem

Let x 2 + y 2 x = u \displaystyle \frac{x^{2} + y^{2}}{x} = u and x 2 + y 2 y = v \displaystyle \frac{x^{2}+y^{2}}{y} = v

( u , v ) ( x , y ) = ( x 2 + y 2 x y ) 2 \displaystyle \left\lvert \frac{\partial(u,v)}{\partial(x,y)} \right\rvert = \left(\frac{x^{2}+y^{2}}{xy}\right)^{2}

So ( x , y ) ( u , v ) = ( x y x 2 + y 2 ) 2 \displaystyle \left\lvert \frac{\partial(x,y)}{\partial(u,v)} \right\rvert = \left(\frac{xy}{x^{2}+y^{2}}\right)^{2}

So after this our integral just becomes a really simple one:

E 1 x y d x d y = T 1 x y ( x y x 2 + y 2 ) 2 d u d v = T x y ( x 2 + y 2 ) 2 d u d v \displaystyle \iint_{E} \frac{1}{xy} dx\,dy = \iint_{T} \frac{1}{xy} \left(\frac{xy}{x^{2}+y^{2}}\right)^{2} du\,dv =\iint_{T}\frac{xy}{(x^{2}+y^{2})^{2}}du\,dv

Where T T is the rectangular region in u v uv plane given by 1 u 2 1\leq u \leq 2 and 1 v 2 1\leq v\leq 2

So we have :- 1 2 1 2 1 u v d u d v \displaystyle \int_{1}^{2}\int_{1}^{2} \frac{1}{uv}\, du\,dv

So A = ( ln ( 2 ) ) 2 \displaystyle A = (\ln(2))^{2}

1000 A = 480 \lfloor{1000A}\rfloor = 480

You are a genius. That’s all I have to say.

Kumudesh Ghosh - 1 year, 1 month ago

That Confused Class ....😂😂😂

Prithwish Mukherjee - 1 year, 1 month ago

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Yuriy Kazakov
Aug 1, 2020

Monte Carlo with Python.

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import  random
n=10000000
f=0
for k in range(n):
  x=random.uniform(0.25,1)
  y=random.uniform(0.25,1)
  if x*x+y*y>x and  x*x+y*y<=2*x and  x*x+y*y<=2*y and   x*x+y*y>y:
    f=f+1/(x*y)
print(f/n*(9/16)*1000) 

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480.29366431811883

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