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Math Help - Questions arising while studying for midterm.

  1. #1
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    Questions arising while studying for midterm.

    I have been going through some practice questions online to prepare for my midterm and ran into a few questions I had problems with but for which there where no explanations for solving.


    1. Find limit of ({\frac{1+81x^5}{x^5+5}})^\frac{1}{4} as x approaches infinity.
    I thought all that would matter in this question is the magnitude of x as I couldn't see any reasonable way to simplify the equation. I answered 1, but the answer is actually 3. What can I do to this problem to make it more manageable?


    2. For the given function f(x) and values of L, y and v > 0 find the largest open interval about y on which the inequality |f(x)-L| < v holds. Then determine the largest value for z > 0 such that 0 < |x-y| < z -> |f(x) - L| < v.
    f(x) = 4x+7,  L = 35, y = 7, v = 0.32

    I worked through all of my textbook questions and never saw something worded like this. I did some work but none of my solutions were close to those that are given. I really have no idea where to start with this question. The answers are: The inequality holds for (6.92, 7.08) and the largest value of z is 0.08.


    3. v = \frac{300x - x^3}{4}  \frac{dv}{dx} = \frac{300 - 3x^2}{4}.

    This is part of a larger word problem; however, every step I followed up to this point matched the process in the answer book. Whenever I took the derivative of v using the quotient rule I would come up with an ugly looking quadratic instead of this elegant solution. What am I doing wrong?
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Vanilla View Post

    1. Find limit of ({\frac{1+81x^5}{x^5+5}})^\frac{1}{4} as x approaches infinity.
    I thought all that would matter in this question is the magnitude of x as I couldn't see any reasonable way to simplify the equation. I answered 1, but the answer is actually 3. What can I do to this problem to make it more manageable?
    Divide top and bottom through by x^5 inside the outermost brackets.

    CB
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  3. #3
    Master Of Puppets
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    Quote Originally Posted by Vanilla View Post


    1. Find limit of ({\frac{1+81x^5}{x^5+5}})^\frac{1}{4} as x approaches infinity.
    I thought all that would matter in this question is the magnitude of x as I couldn't see any reasonable way to simplify the equation. I answered 1, but the answer is actually 3. What can I do to this problem to make it more manageable?
    I have a love/hate relationship with limits.

    \displaystyle\lim_{x \to \infty}\sqrt[4]{{\frac{1+81x^5}{x^5+5}}}

    \displaystyle=\lim_{x \to \infty}\sqrt[4]{{\frac{(1+81x^5)\times \frac{1}{x^5}}{(x^5+5)\times \frac{1}{x^5}}}}

    \displaystyle=\lim_{x \to \infty}\sqrt[4]{{\frac{\frac{1}{x^5}+81}{1+\frac{5}{x^5}}}}

    \displaystyle=\sqrt[4]{{\frac{\frac{1}{\infty^5}+81}{1+\frac{5}{\infty^5  }}}}

    \displaystyle=\sqrt[4]{{\frac{0+81}{1+0}}}

    =\sqrt[4]{81} = 3
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