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Calculus Level 5

A curve passes through ( 2 , 2 ) (-2,-2) and its slope at the point ( x , y ) (x,y) is given by 1 x x 2 1 \dfrac 1 {x\sqrt{x^{2}-1}} . Which of the following points the curve also passes through?

  • A: ( 2 3 , π 6 2 ) \ \ \left(-\dfrac{2}{\sqrt{3}}, \ -\dfrac{\pi}{6}-2\right)
  • B: ( 2 3 , π 6 2 ) \ \ \left(-\dfrac{2}{\sqrt{3}}, \ \dfrac{\pi}{6}-2\right)
  • C: ( 2 , π 12 2 ) \ \ \left(-\sqrt{2}, \ -\dfrac{\pi}{12}-2\right)
  • D: ( 2 , π 12 2 ) \ \ \left(-\sqrt{2}, \ \dfrac{\pi}{12}-2\right)
D only A only B only C only A and D only C and D only B and D only A and C only

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1 solution

Guilherme Niedu
Mar 15, 2017

d y d x = 1 x x 2 1 \large \displaystyle \frac{dy}{dx} = \frac{1}{x\sqrt{x^2-1}}

y = 1 x x 2 1 d x \large \displaystyle y = \int \frac{1}{x\sqrt{x^2-1}} dx

u = x 2 1 x = ± u 2 + 1 \large \displaystyle \color{#3D99F6} u = \sqrt{x^2-1} \rightarrow x = \pm \sqrt{u^2+1}

d u = x x 2 1 d x d u = ± u 2 + 1 u d x \large \displaystyle \color{#3D99F6} du = \frac{x}{\sqrt{x^2-1}} dx \rightarrow du = \pm \frac{\sqrt{u^2+1}}{u} dx

y = 1 u u 2 + 1 u u 2 + 1 d u \large \displaystyle y = \int \frac{1}{u\sqrt{u^2+1}} \cdot \frac{u}{\sqrt{u^2+1}} du

y = 1 u 2 + 1 d u \large \displaystyle y = \int \frac{1}{u^2+1}du

y = arctan ( u ) + C \large \displaystyle y = \arctan(u) + C

y = arctan ( x 2 1 ) + C \large \displaystyle y = \arctan(\sqrt{x^2-1}) + C

Since it passes through the point ( 2 , 2 ) (-2, -2) :

2 = arctan ( 3 ) + C \large \displaystyle -2 = \arctan(\sqrt{3}) + C

2 = π 3 + C \large \displaystyle -2 = \frac{\pi}{3} + C

C = 2 π 3 \large \displaystyle \color{#3D99F6} C = -2 - \frac{\pi}{3}

So:

y = arctan ( x 2 1 ) 2 π 3 \color{#3D99F6} \boxed{ \large \displaystyle y = \arctan(\sqrt{x^2-1}) -2 - \frac{\pi}{3} }

Which passes through points ( A ) (A) and ( C ) (C) only.

A Really tricky problem @Rohith M.Athreya . i forgot taking modulus sign and hence ended with BD . Then when solved using Modulus i got AC But i was too late :P

Prakhar Bindal - 4 years, 2 months ago

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Yes! that's why i posted :P

Rohith M.Athreya - 4 years, 2 months ago

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Yep!! I saw the title and knew there was something tricky !

Sumanth R Hegde - 4 years, 2 months ago

In your third line u took, x x as positive?

Rohith M.Athreya - 4 years, 2 months ago

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You could take it as negative, it will appear two negative signs (one for u u and other for d u du ) that will cancel out

Guilherme Niedu - 4 years, 2 months ago

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yes!

just thought I'd ask for clarity!

thanks!

Rohith M.Athreya - 4 years, 2 months ago

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@Rohith M.Athreya Perfect! Thanks. I've edited to be more clear.

Guilherme Niedu - 4 years, 2 months ago

The question is overrated I think

Md Zuhair - 4 years, 2 months ago

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or u are too good

Rohith M.Athreya - 4 years, 2 months ago

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No i dont think, But this integration is much easy, and after that Getting the c, And then Putting the Options, we get it

Md Zuhair - 4 years, 2 months ago

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@Md Zuhair It is very easy indeed and just as easy to miss something

I had it pegged at lvl4 but due to low correct rate, it got readjusted to lvl5

Rohith M.Athreya - 4 years, 2 months ago

awsome one. Forgotten modulus sign while taking under root .. Really a good question keep posting such stuffs

Nivedit Jain - 4 years, 2 months ago

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It was initially lvl 5. But i think it is lvl 4, As The Question is not so tough , Specially the integration

Md Zuhair - 4 years, 2 months ago

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integration is not tough at all i agree but thing i forgot is to take modulus. otherwise and easy stroke

Nivedit Jain - 4 years, 2 months ago

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@Nivedit Jain It is the common mistake ...

Md Zuhair - 4 years, 2 months ago

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@Md Zuhair ya it is that s why i liked it as it point out to my common mistake

Nivedit Jain - 4 years, 2 months ago

I took it out as a "plus-minus"

Guilherme Niedu - 4 years, 2 months ago

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ya i have seem it but i have done a diffrent way

Nivedit Jain - 4 years, 2 months ago

If I put x= secØ, then...... Well my answer is not matching then.

Rohit Vijayvargiya - 3 years, 4 months ago

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Yeah I tried that too. But I ended up solving the integral as arcsec(x) which was undefined at -2.

Tristan Goodman - 1 year, 10 months ago

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