1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rationalizing Substitutions

  1. Jun 27, 2006 #1
    Okay, Im having trouble with the following, is it as hard as I think or..?There are no examples or anything in the book concerning rationalizing trig substitutions:

    [tex]\int \frac{cosx}{sin^2x +sinx}dx[/tex]

    thanks
     
  2. jcsd
  3. Jun 27, 2006 #2

    0rthodontist

    User Avatar
    Science Advisor

    let u = sin x
     
  4. Jun 27, 2006 #3

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Usually, there are several obvious things to try. So try them, instead of simply mulling about wondering if they'll work.

    Without any ingenuity at all, you should be able to come up with

    u = cos x
    u = sin x
    u = sin² x
    u = sin² x + sin x
    [itex]u = (\cos x) / (\sin^2 x + \sin x)[/itex]

    as things to try.
     
  5. Jun 27, 2006 #4

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    The substitution [itex]u=\tan(\frac{x}{2})[/itex] takes care of just about every contrived exercise integral you'll ever meet. :smile:
     
  6. Jun 27, 2006 #5
    so let [tex]u=tan \frac{x}{2}[/tex]
    [tex]x=2tan^-^1u[/tex]
    [tex]dx=\frac {2du}{1+u^2}[/tex]

    and then find replacements for sinx and cosx using "u"?
     
  7. Jun 27, 2006 #6

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    That's correct!
    A helpful step is to rewrite your integrand as:
    [tex]\frac{\cos(x)}{\sin^{2}(x)+\sin(x)}=\frac{\cos^{2}(\frac{x}{2})-\sin^{2}(\frac{x}{2})}{4\sin^{2}(\frac{x}{2})\cos^{2}(\frac{x}{2})+2\sin(\frac{x}{2})\cos(\frac{x}{2})}=\frac{1-\tan^{2}(\frac{x}{2})}{4\sin^{2}(\frac{x}{2})+2\tan(\frac{x}{2})}[/tex]
    However, it might well be that some of the substitutions mentioned by Hurkyl are easier.
     
    Last edited: Jun 27, 2006
  8. Jun 27, 2006 #7

    StatusX

    User Avatar
    Homework Helper

    u=sin(x) is the obvious and simplest choice in this case. Obvious because the integrand is clearly in the form f(sin(x))*d(sin(x))/dx. And simple because the resulting integrand is a rational function, which can be integrated by partial fractions.

    If there were no other obvious substitution, then it would be a good idea to use arlidno's suggestion, because this is guaranteed to turn a rational function in any trigonometric function into a rational function in u.
     
    Last edited: Jun 27, 2006
  9. Jun 27, 2006 #8
    Wait, I just tried u=sinx, and I got an answer pretty easily, is this substitution suppose to be easier?
     
  10. Jun 27, 2006 #9
    yea..thanks guys
     
  11. Jun 27, 2006 #10
    how about for this one:[tex]\int ln(x^2-x+2)dx[/tex]

    im guessing im going to have to use partial fractions and integrateby parts. Is there any easier way to figure what im going to substitute, I always have trouble figuring out what to sub, usually just trial and error
     
  12. Jun 27, 2006 #11

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you practice you will eventually learn to recognize the patterns in integrands, and your intuition will help you pick out what is most likely to be useful, and try those first.

    Incidentally, this integral is rather straightforward, if you remember how to manipulate logarithms.
     
  13. Jun 27, 2006 #12
    i have been praticing, I still find it hard though.
     
  14. Jun 27, 2006 #13

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It changes eventually. Experience really does make a difference.
     
  15. Jun 27, 2006 #14
    arildno (or anyone else for that matter), can you elaborate on this comment please, or maybe point me in the direction to find an explaination myself. That seems like a very interesting concept and I think I might be able to apply that to my studies as well.

    Thanks
     
  16. Jun 27, 2006 #15

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    I was exaggerating a bit; read StatusX's post for a more accurate description.
     
  17. Jun 27, 2006 #16
    [tex]\int x cscx cotx dx[/tex]..I simplified it a little and got

    [tex]\int \frac{xcosx}{sin^2x}dx[/tex]..im kinda stuck here, is it a partial fraction substitution or integration by parts, can anyone tell me if im on the right track?
     
  18. Jun 27, 2006 #17

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    And, just adding to Hurkyl's post:
    You DEVELOP that intuition; it can simply be regarded as the flashes of insight gained from condensed experience&practice, like a grandmaster in chess who might tell at a glance what might work and what won't, without doing the tedious calculations.
     
  19. Jun 27, 2006 #18

    0rthodontist

    User Avatar
    Science Advisor

    If you practice by trying to do them mentally instead of on paper, you'll get better faster. Each problem might take you twice as long at first because you have to keep repeating the expression to yourself, but it will force you to be more aware of what you're doing.
     
  20. Jun 27, 2006 #19
    yeah, I think im going to go back and practise everyhting i learned before i go further..Ive only done like 5-10 problems in each topic...probly a good idea;)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?