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Integrals (u-substitution)

  1. Jul 5, 2011 #1
    1. The problem statement, all variables and given/known data
    67ph6q.jpg

    2. Relevant equations

    3. The attempt at a solution

    I don't understand what exactly is going on here. They let [itex]u=(1+x^{2})[/itex], so that leaves them with this:
    [tex]\int \frac{x}{u^{2}}dx[/tex]

    The derivative of [itex](1+x^{2})[/itex] is simply [itex]2x[/itex]. And so:
    [tex]\frac{du}{dx} = 2x \rightarrow du = 2xdx \rightarrow dx=\frac{du}{2x}[/tex]

    So now, substituting in my new dx, I get:

    [tex]\int \frac{x}{u^{2}2x}du[/tex]

    So, is that x simply canceling out here? Is that the idea?
    Which leaves me with:
    [tex]\int \frac{u^{-2}}{2}du[/tex]
    [tex]-\frac{1}{2u}[/tex]
    Re-substituting u I get:
    [tex]-\frac{1}{2(x^{2}+1)} + C[/tex]

    With that being said, how do you know that the x will cancel? How are you even supposed to know that this approach will work? Is there some sort of proof to this idea, my book does not have it.
     
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  3. Jul 5, 2011 #2

    rock.freak667

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    Yes that is the basic idea of it.

    We know it will work since d/dx(x2+1) = 2x, meaning that in the integrand the x in the numerator will cancel out.
     
  4. Jul 5, 2011 #3

    gb7nash

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    That's pretty much the idea. Another way to look at it is:

    [itex]u=(1+x^{2})[/itex] , [itex]du= 2x dx[/itex] , so [itex]\frac{1}{2}du=xdx[/itex].

    Now substituting in, you have [itex]\int \frac{1}{u^{2}} \frac{1}{2}du[/itex] = ...
     
    Last edited: Jul 5, 2011
  5. Jul 5, 2011 #4

    micromass

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    Yes, that's it. You know that [itex]u=x^2+1[/itex], so deriving both sides gives you [itex]du=2xdx[/itex], thus [itex]dx=\frac{du}{2x}[/itex]. And then you can substitute it in the integral.


    Basically, you don't know if the approach will work, you'll only know it by trying. You'll need some experience with these kind of thingies. If you've solved lots of integrals then you know immediately a few substitutions that you could try. But you'll still need to try them to see if they will really work out.

    The same thing happens with the integral

    [tex]\int{x\sqrt{x^2+1}dx}[/tex]

    when confronted with a root, I always try the substitution [itex]u=\sqrt{x^2+1}[/itex] first. Sometimes it works, sometimes it doesn't. With this substitution, you get [itex]u^2=x^2+1[/itex] and by deriving both sides:

    [tex]2udu=2xdx[/tex]

    thus [itex]dx=\frac{udu}{x}[/itex]

    and you see that the x will cancel again. Note, if there wasn't an x before the root, then it wouldn't have worked. And if there wasn't an x in the numerator of your integral, then it wouldn't have worked...
     
  6. Jul 5, 2011 #5
    gb7nash,

    [itex]du= 2x dx[/itex] , so [itex]\frac{1}{2}du=dx[/itex]
    Where did the x go in this example?

    [itex]du=2xdx[/itex]

    [itex]\frac{1}{2}du=xdx[/itex]

    [itex]\frac{1}{2x}du=dx[/itex] ?
     
  7. Jul 5, 2011 #6

    gb7nash

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    Typo. Thanks.
     
  8. Jul 5, 2011 #7
    Ah okay. The way you phrased it, I thought you were doing something different than me.

    Thanks for all of the help everyone.
     
  9. Jul 5, 2011 #8

    S_Happens

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    Personally after finiding du I would solve for xdx instead of for dx in this case, since that is what you have in the integral. Obvioulsy you can take the couple extra steps to see that it cancels out, but I don't see the reason.

    At the point where you have 1/2du = xdx, I would simply make the substitution back into the integral, rather than making the extra steps that give you the same result.
     
  10. Jul 5, 2011 #9
    Related to this question:

    [tex]\int sec^{3}(x)tan(x) dx[/tex]

    [itex]u=sec(x)[/itex], and so, [itex]\frac{du}{dx}=sec(x)tan(x)[/itex], and [tex]dx=\frac{du}{sec(x)tan(x)}[/tex]

    [tex]\int u^{3}tan(x) dx[/tex]
    [tex]\int \frac{u^{3}tan(x)}{sec(x)tan(x)} du[/tex]
    [tex]\int \frac{u^{3}}{sec(x)} du[/tex]

    I have no idea what to do with this one? Can I put my u=sec back in and try again now?

    Edit: Na, that just gets me back to where I started.

    Oh wait, because u is equal to sec, can I just call that [itex]\frac{u^{3}}{u}[/itex] ?
     
  11. Jul 5, 2011 #10

    rock.freak667

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    You'd get

    [tex]\int \frac{u^3}{u} du = \int u^2 du[/tex]
     
  12. Jul 5, 2011 #11
    Yeah! I see what you did there (above edit). Fantastic stuff. I haven't had to actually think about math for a while, this is easily the funnest section I have done to date.
     
  13. Jul 5, 2011 #12

    vela

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    One thing you should always check is if the integral is of or can be tweaked into the form
    [tex]\int [f(x)]^n f'(x)\, dx[/tex]
    If it is, the substitution u=f(x) will work, and the answer will be
    [tex]\int [f(x)]^n f'(x)\, dx = \left\{
    \begin{array}{lc}
    \frac{[f(x)]^{n+1}}{n+1}+c & n\ne -1 \\
    \\
    \log \lvert f(x) \rvert+c & n=-1
    \end{array}
    \right.[/tex]
    In your problem, you'd guess f(x)=x2+1 so that f'(x)=2x, and it indeed works out:
    [tex]\int \frac{x}{(x^2+1)^2}\,dx = \frac{1}{2}\int (x^2+1)^{-2} (2x) \,dx[/tex]
    If you can differentiate in your head, you can often an the integral fits the pattern by inspection. It's a neat trick to know mostly so you can screw with your peers when you look at an integral and say, "Oh, the answer is obviously..." and then write the answer down without showing any work. They'll be in awe of your mathematical prowess. :wink:
     
  14. Jul 5, 2011 #13

    vela

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    For your latest problem, you'd say
    [tex]\int \sec^3 x\tan x \,dx = \int (\sec x)^2 (\sec x \tan x)\,dx = \cdots[/tex]
     
  15. Jul 5, 2011 #14
    I came up with:

    [tex]\frac{sec^3(x)}{3}+C[/tex]


    @Vela,
    That is basically looking to see if it is a backwards chain rule correct?
     
  16. Jul 5, 2011 #15
    [tex]\int \frac{x^{2}}{\sqrt(1-x)}dx[/tex]

    I let [itex]u=\sqrt(1-x)[/itex], and [itex]dx = \frac{2\sqrt(1-x)du}{-1}[/itex]

    So I came to this guy here:

    [tex]\int \frac{2x^{2}\sqrt(1-x)}{-u}du[/tex]

    But now if I resubstitute the [itex]\sqrt(1-x)[/itex] back in for u to cancel, I am still stuck with that x, and du at the end.
     
  17. Jul 5, 2011 #16

    micromass

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    It's easier not to work with the square roots:

    IF [itex]u=\sqrt{1-x}[/itex], then [itex]u^2=1-x[/itex], thus [itex]2udu=-dx[/itex].

    Furthermore, [itex]x=1-u^2[/itex], thus [itex]x^2=(1-u^2)^2[/itex].

    Now you can substitute everything in the integral.
     
  18. Jul 5, 2011 #17
    Oh, I didn't even think about squaring both sides of the u-sub equation!
    Would that work with other things too? For instance, suppose I say that u=MNOP, and then in the integral I see that there is a M/Mu, Could I multiply both sides by M, to get Mu = M^2NOP, and sub it in to cancel the M in the numerator and such? Basically, is any normal operation okay there?

    I'm working on it!
     
  19. Jul 5, 2011 #18

    vela

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    Yup.
    Which, since [itex]u=\sqrt{1-x}[/itex], gives you [itex]dx=-2u\,du[/itex]. Then you need to express x in terms of u to finish off the substitution, which should give you the same thing micromass got.
     
  20. Jul 5, 2011 #19
    Okay, I am giving this problem a shot now:

    [tex]\int \frac{x^{2}}{\sqrt{1-x}}dx[/tex]
    [tex]u=\sqrt{1-x}[/tex]
    [tex]u^{2}=1-x[/tex]
    [tex]x=1-u^{2}[/tex]

    [tex]\frac{du}{dx}=\frac{-1}{2\sqrt{1-x}}[/tex]

    Since [itex]\sqrt{1-x} = u[/itex], then

    [tex]\frac{du}{dx}=\frac{-1}{2u}[/tex]
    [tex]dx = -2udu[/tex]

    and so here is the squiggly guy:

    [tex]\int \frac{-2ux^{2}}{u}du[/tex]

    and since [itex]x=1-u^{2}[/itex]

    [itex]x^{2}=1-2u^{2}+u^{4}[/itex]

    and so, putting all that back into the integrand:

    [tex]\int \frac{-2u(1-2u^{2}+u^{4})}{u}du[/tex]

    Pair of u's cancel out

    [tex]\int -2(1-2u^{2}+u^{4})du[/tex]

    [tex]\int -2+4u^{2}-2u^{4}du[/tex]

    Which is something that I can integrate easily....

    [tex]-2u+\frac{4u^{3}}{3}-\frac{4u^{5}}{5}[/tex]

    To which, back go my original u's to get...

    [tex]-2(\sqrt{1-x})+\frac{4(\sqrt{1-x})^{3}}{3}-\frac{4(\sqrt{1-x})^{5}}{5} + C[/tex]

    Which is a nightmare to simplify, that can't be right?
     
  21. Jul 5, 2011 #20

    vela

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    I'd leave it as it. You can differentiate it and see if you recover the integrand.
     
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