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Homework Help: Does this make sense

  1. Sep 26, 2005 #1
    in int[6/(x^2+3)^2] i use 3tanU=x, dx=3sec(u)^2 du
    using that i get to .... int[1+cos2u]= u + sin2u/2 = u +sin u cos u... but subing back in with trig doesnt give me the right answer... any help with this method?
     
  2. jcsd
  3. Sep 26, 2005 #2
    [tex]I = \int {\frac{6}{{\left( {x^2 + 3} \right)^2 }}dx} [/tex]

    Instead of setting x = 3tan(u) try...

    [tex]
    x = \sqrt 3 \tan \theta \Rightarrow dx = \sqrt 3 \sec ^2 \theta d\theta
    [/tex]
     
  4. Sep 26, 2005 #3
    darn it, after looking at my work i was afraid someone would say that... as i didnt relize that would be better until i finished it and noticed it was coming out right.... does that mean you cant do it my way:-/
     
  5. Sep 26, 2005 #4
    There's really no reason why it can't be done your way. It's just that some substitutions will require you to evaluate much more complicated integrals than if you were to use a substitution which obviously simplifies the integrand.
     
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