Very strange shape (2)

Calculus Level 4

There are shapes defined by the overlapping region of 2 plots:

1 x = y \dfrac {1}{x} = y

x 2 + y 2 = 4 x^2 + y^2 = 4

If the sum of the areas of these shapes is β \beta , find β \lfloor{\beta}\rfloor .


The answer is 1.

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2 solutions

Just a quick outline ...

Focussing on the 'shape' in the first quadrant, the points of intersection are the positive roots of x 4 4 x 2 + 1 = 0 x^{4} - 4*x^{2} + 1 = 0 , which come out to be a = 2 3 a = \sqrt{2 - \sqrt{3}} and b = 2 + 3 b = \sqrt{2 + \sqrt{3}} .

We then need to calculate

a b ( 4 x 2 1 x ) d x \int_{a}^{b} (\sqrt{4 - x^{2}} - \frac{1}{x}) dx .

For the first term we'll need to make a trig substitution of x = 2 sin ( θ ) x = 2\sin(\theta) . Going through the motions, we find that the integral comes out to

2 arcsin ( x 2 ) + x 2 4 x 2 ln x 2*\arcsin(\frac{x}{2}) + \frac{x}{2}\sqrt{4 - x^{2}} - \ln{x} ,

evaluated from a a to b b . The area of this 'upper' shape is then

2 π 3 ln ( 2 + 3 ) \dfrac{2\pi}{3} - \ln(2 + \sqrt{3}) ,

which by symmetry will also be the area of the 'lower' shape. To 3 3 decimal places the combined area is then 1.555 1.555 , leaving us with a final answer of β = 1 \lfloor \beta \rfloor = \boxed{1} .

Very nice solution. @Aditya Raut , if you want a proper solution.

Sharky Kesa - 6 years, 9 months ago

best solution

Mardokay Mosazghi - 6 years, 9 months ago
Aditya Raut
Aug 23, 2014

Too much bashable, anyone with a little logic will bash this the same way as I did.

We know how the 2 graphs look, 1 is a hyperbola and other is a circle. They intersect as shown in the following figure img img

Now, area of the circle is π r 2 = 4 π \pi r^2 = 4\pi .

Hence the area of one quarter circle is π \pi .

And area of one part of the intersection (2 parts as you see) ,is even less than 1 3 \dfrac{1}{3} of the area of the quarter circle, so the area shaded is less than 2 π 3 = 2.0943951023931953 \dfrac{2\pi }{3} = 2.0943951023931953 , so answer will be either 2 2 or 1 1 .

But the area is too less than the 1 3 \frac{1}{3} of quarter circle, so it won't be greater than 2.

Answer 1 \boxed{1}

Bashed badly, but waiting for elegant way. Sorry @Sharky Kesa .

Meh. Least you got an answer.

Sharky Kesa - 6 years, 9 months ago

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How can we get the exact area?That's what I'm curious about

Bogdan Simeonov - 6 years, 9 months ago

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The exact area is irrational so it can be written in the form of a definite integral other than thag the notion ⌊β⌋ means int(β) which implies that you should only write the integer separate from it's decimal

Kareem Khalil - 6 years, 9 months ago

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@Kareem Khalil Yes, I know why the greatest integer function was used.Brian Charlesworth put a solution of finding the exact value and that's what I was interested in

Bogdan Simeonov - 6 years, 9 months ago

The distance between the two points of intersection of the hyperbola and the circle is equivalent to 2.. By solving for the angle opposite this side with radius 4, angle is equivalent to 29pi/180=0.50536.. Area of the sector would be 1/2(4^2)(0.50536)=4.042884, subtracted by the area of the triangle which is equal to 1/2(4^2)(sin 0.50536)=3.87298 is 4.042884-3.87298=0.169904.. Since there are four regions to be considered, multiplying it by 4 will yield 0.697616..

Mark Vincent Mamigo - 6 years, 6 months ago

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