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Homework Help: Help with this Calculus problem

  1. Jan 24, 2012 #1
    1. The problem statement, all variables and given/known data
    The Fresnel function is given as S(x) = ∫sin(3πt^2)dt from 0 to x. Find the limit as x approaches 0 of S(x)/4x^3

    2. Relevant equations

    3. The attempt at a solution
    I took the derivative of the S(x) function to be able to plug x in. I then used L'Hospital's rule after getting 0/0. I took the derivative a second time after getting 0/0 again. My final answer was π/2, which is wrong. The other answer choices are π/4, 3π/2, 1/2, and 1/4.

    sin(3πx^2)/4x^3 ; took derivative of top and bottom and got (6πx)(cos(3πx^2)/12x. Plugged in 0 and got 0/0. Took derivative of top and bottom again and got, (cos3πx^2)(6π)+(6πx)(-sin(3πx^2)(6πx). Plugged in 0 and got π/2 as final answer. Where did I go wrong and what is the right answer?
  2. jcsd
  3. Jan 24, 2012 #2
    When you took the derivative of S(x) = ∫sin(3πt^2)dt and plugged x in to get sin(3πx^2), that counts as differentiating right? Did you do the same to the denominator? You claimed you started with sin(3πx^2)/4x^3, but shouldn't you start with sin(3πx^2)/12x^2 ?
  4. Jan 24, 2012 #3
    OMFG. Derivative of 4x^3 is NOT 12x. WOW....I HATE WHEN I MAKE STUPID MISTAKES.....Answer is pi/4. FML. Lost five points on my homework because of THAT stupid carelessness.
  5. Jan 24, 2012 #4
    :D. It's moments like these that make you less prone to error in the future :P, atleast I find.
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