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Homework Help: Two integral problems including e and sin

  1. Sep 6, 2011 #1
    1. ∫ 6e^(-y/4) dy

    2. ∫ [ sin(pi x) - (3/x) ] dx


    Any help with either of these would be great. I'm teaching myself integrals for Physics C and i have absolutely no clue what to do with these.

    Thanks in advance
     
  2. jcsd
  3. Sep 6, 2011 #2
    For the first one: factor ou the 6 and find a suitable substitution to bring the integral back to

    [tex]\int{e^ydy}[/tex]
     
  4. Sep 7, 2011 #3
    For the second one you can separate it in to two integrals, sin(pi x) dx and -3/x dx
     
    Last edited: Sep 7, 2011
  5. Sep 7, 2011 #4
    Thanks, I figured out the first one.

    But am still confused with this one. So far i have :

    -1/4 ∫ cos(pi x) - 3Ln| x|

    These were actually both definite integrals I was supposed to evaluate..so when I evaluated -1/4 ∫ cos(pi x) - 3Ln| x| from 10 to 2 I was within 6 tenths of the right answer. Is the answer sheet wrong or what did i do wrong?

    Also, this may be a silly question but am I supposed to use pi as in 3.14 or 180
     
  6. Sep 7, 2011 #5
    How did you get -1/4 ∫cos(pi x) ?

    x is probably in radians, so you'll use 3.14 instead of 180 for degrees.
     
    Last edited: Sep 7, 2011
  7. Sep 7, 2011 #6
    I think I just wrote -1/4 instead of -1/pi by accident. But I got help from a teacher so it's all good now.

    Thanks for the help everyone.
     
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