Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Aarrr, help?

  1. May 23, 2004 #1
    I need help with these problems:

    (1) STan^-1(y)dy

    and

    (2) Ssin^-1(x)dx

    (S=integral)

    I can't seem to figure out the trigonometric formula for integration. A little help or hint would be nice. Thanks!
     
  2. jcsd
  3. May 23, 2004 #2

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Are you talking about arctan and arcsine (the inverse functions)?
    If so, use integration by parts in both cases, with 1 as the function you integrate.
     
  4. May 23, 2004 #3
    Do you have to use a trigonometric method to integrate those? I think they could both be readily solved with integration by parts.

    Edit: arildno beat me to it :)

    If you need help with integration by parts, let us know. It works really well in this case.
     
  5. May 23, 2004 #4
    Um, arildno or TALewis can you show me how to do it using integration by parts. I just learnt this relatively recently and haven't got the core concepts down. If you could show me, it would be much appreciated.

    Thanks! :approve:
     
  6. May 23, 2004 #5

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  7. May 23, 2004 #6
    Aasrr, i was born on a pirate ship! ok now that i got that out of my system...

    i'm assuming for the problem you mean:

    1 )[tex] \int arctan(y) [/tex] [tex] dy [/tex]

    and

    2)[tex]\int arcsin(x) [/tex] [tex] dx [/tex]

    (thats the inverse tangent and inverse sine functions respectively). In that case you must perfomr a substitution before using integration by parts:

    1)

    [tex] \theta = arctan(y) [/tex]

    [tex] tan(\theta) = y [/tex]

    [tex] dy = sec^2(\theta) [/tex] [tex] d\theta [/tex]

    So the integral becomes:

    [tex] \int \theta*sec^(\theta) [/tex] [tex] d\theta [/tex]

    which using integration by parts gives:

    [tex] \int u [/tex] [tex] dv = u*v-\int v [/tex] [tex] du [/tex]

    with

    [tex] u = \theta [/tex]
    [tex] dv = sec^2(\theta) [/tex]

    so

    [tex] \int \theta*sec^2(\theta) [/tex] [tex] d\theta = \theta*tan(\theta)-\int tan(\theta) [/tex] [tex] d\theta [/tex]

    [tex] = \theta*tan(\theta) + ln|cos \theta| [/tex]

    So substituting for theta we get

    [tex] \int arctan (y) [/tex] [tex] dy = yarctan(y) + ln|cos(arctan y)| [/tex]
    [tex] =y*arctan(y) + ln|\frac{1}{\sqrt{1+y^2}}| [/tex]
    [tex] =y*arctan(y) - \frac{1}{2}ln|y^2+1| [/tex]
    [tex] =y*arctan(y) - \frac{1}{2}ln(y^2 + 1) [/tex]

    U can use a similar substitution in the second problem (simply construct a right triangle, substitute theta in, integrate, and the substitute x back in.

    Note: LaTeX still hates me...eventually i'll get it to look right...
     
    Last edited: May 23, 2004
  8. May 23, 2004 #7
    franznietzsche, I am curious why you made that substitution. I approach the problem this way. We want to evaluate:

    [tex]\int \tan^{-1}y\,dy = I[/tex]

    I use capital I to denote the integral to be evaluated.

    I choose the following directly from the integral:

    [tex]
    \begin{align*}
    u &= \tan^{-1}y & du &= \frac{dy}{1+y^2}\\
    dv &= dy & v &= y
    \end{align}
    [/tex]

    Integration by parts is given by:

    [tex]\int u\,dv = uv-\int v\,du[/tex]

    So I have:

    [tex]
    \begin{align*}
    I&=y\tan^{-1}y - \int \frac{y\,dy}{1+y^2}\\
    \therefore \int \tan^{-1}y\,dy&=y\tan^{-1}y - \frac{1}{2}\ln(1+y^2)
    \end{align}
    [/tex]
     
    Last edited: May 23, 2004
  9. May 23, 2004 #8

    I suppose some combination of liking trig substitutions inherently and not wanting to differetiate [tex] arctan y [/tex] simply because i didn't know it offhand and didn't want to look it up.
     
  10. May 23, 2004 #9
    Hm, I agree. If you don't have the derivative of arctan memorized, you have to look it up. And while you're looking things up, you might as well just use an integral table anyway.
     
  11. May 23, 2004 #10
    yup, exactly.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Aarrr, help?
  1. Help with integration (Replies: 3)

  2. Help with an integral (Replies: 3)

  3. Proof help (Replies: 1)

  4. Help with an integral (Replies: 0)

  5. Help with a limit (Replies: 5)

Loading...