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Homework Help: Integration help

  1. Dec 14, 2008 #1
    1. The problem statement, all variables and given/known data

    integrate this

    9x³/sqrt(1+x²)

    I know im suppose to substitute using: x = tan x
    dx = sec²x

    but thats as far as i could go. its the 9x³ that confuses me on what i should do.
    thank you for your help in advance.
     
    Last edited: Dec 14, 2008
  2. jcsd
  3. Dec 14, 2008 #2
    You should not be substituting x = tan x. You should be substituting x = tan u. Then, dx = sec2u du. So substitute tan u everywhere there is a x, and sec2u du for dx to get a complete integral in u.
     
  4. Dec 15, 2008 #3

    dextercioby

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    [tex] x= \sinh u [/tex] as a substitution should do the trick.
     
  5. Dec 15, 2008 #4

    Dick

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    You don't need trig at all. u=1+x^2 will work fine. Try it.
     
  6. Dec 15, 2008 #5
    thanks guys for all the help really appreciate it
     
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