Beautiful Area Between The Lines And The Curve

Calculus Level 4

In the above graph, the red curve is a part of the curve x 2 y 2 = 1 x^2-y^2=1 (mostly in the first quadrant). A point A A lies on this red curve and is connected to the origin by the green line segment.

If the x x -coordinate of point A A equals e 2016 + e 2016 2 \dfrac{e^{2016}+e^{-2016}}{2} , find the area bounded by the red curve, green line segment and the black x x -axis in the first quadrant.


The answer is 1008.0.

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1 solution

Let any point on the hyperbola be ( x , x 2 1 ) (x, \sqrt{x^{2}-1} ) .
The area under the line drawn from origin to the point is
Δ 1 = 1 2 × x × y = x x 2 1 2 \Delta_{1} = \dfrac{1}{2} \times x \times y = \dfrac{x\sqrt{x^{2}-1}}{2}

The area under the hyperbola is given by,

Δ 2 = 1 x t 2 1 d t \Delta_{2} = \displaystyle \int_{1}^{x}\sqrt{t^{2}-1} dt

It can be derived very easily integrating by parts,
t 2 a 2 d t = t t 2 a 2 a 2 ln ( t + t 2 a 2 ) 2 \displaystyle \int \sqrt{t^{2}-a^{2}}dt = \dfrac{t\sqrt{t^{2}-a^{2}} - a^{2}\ln\left( t + \sqrt{t^{2}-a^{2}} \right)}{2}

Putting a=1 and substituting the limits,

Δ 2 = x x 2 1 ln ( x + x 2 1 ) 2 \therefore \Delta_{2} = \dfrac{x\sqrt{x^{2}-1} - \ln\left( x + \sqrt{x^{2}-1}\right)}{2}

Required area is,
Δ r e q = Δ 1 Δ 2 \Delta_{req} = \Delta_{1} - \Delta_{2}
Δ r e q = ln ( x + x 2 1 ) 2 \therefore \Delta_{req} = \dfrac{\ln \left( x + \sqrt{x^{2}-1}\right)}{2}
For the given question, substitute x = e 2016 + e 2016 2 x = \dfrac{e^{2016} + e^{-2016}}{2} , and Δ r e q \Delta_{req} comes out to be 1008.

You could've used hyperbolic functions. The given form of the expression hints this. Plus, with hyperbolic function identities, the integration part becomes much easier.

A Former Brilliant Member - 5 years, 2 months ago

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I know, but that doesn't explain much to a person not familiar with hyperbolic functions.

A Former Brilliant Member - 5 years, 2 months ago

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it can be really easily done just like vignesh by neglecting the 1 in addition as x>>>1 i did just like it very easily

A Former Brilliant Member - 4 years, 2 months ago

Yes, I was surprised by Vighu's solution not using hyperbolic function. :upside down face:

Nihar Mahajan - 5 years, 2 months ago

Isn't the answer sinh ( 2 ) 4 + 2015 2 \frac{\sinh(2)}{4}+\frac{2015}{2} ? So it should not be exactly 1008 right?

Arghyadeep Chatterjee - 4 months ago

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