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Math Help - L'hospital rule confusion

  1. #1
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    L'hospital rule confusion

    I have the following limit:

    lim t->0 (e^t - 1) / t ^ 3

    I use L'hospital's rule to keep differentiating the numerator and denominator, and I end up with 1/6. But, according to my textbook, the correct answer is infinity. Does anyone see what I am doing wrong here?
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  2. #2
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    \lim_{t\to 0}\frac{e^t - 1}{t} is the derivative of e^t at t=0 by definition. Which is e^0 = 1.

    That means,
    \lim_{t\to 0}\frac{e^t-1}{t^3} = \lim_{t\to 0}\frac{e^t - 1}{t}\cdot \frac{1}{t^2} = \infty
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  3. #3
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    What you're doing wrong

    Quote Originally Posted by asiler View Post
    I have the following limit:

    lim t->0 (e^t - 1) / t ^ 3

    I use L'hospital's rule to keep differentiating the numerator and denominator, and I end up with 1/6. But, according to my textbook, the correct answer is infinity. Does anyone see what I am doing wrong here?
    OK, first review under what circumstances l'H can be used. Done it? Right then ......

    After the first differentiation you no longer have an indeterminant form of the type 0/0. You're mistake was ........

    in the continued use of l'H beyond the first differentiation.
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