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Math Help - Limits - find parameters γ, δ

  1. #1
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    Limits - find parameters γ, δ

    Made a new thread for this as instructed by Plato. On to the limit itself

    \lim _{x\mapsto 0} \frac {\gamma + \delta \sin {x}}{x^2} = 3

    I found this in a book with exercises and although Plato mentioned it has no solutions the book says they're γ = 6 and δ = -6 . Also I double checked it and there is no x in front of γ .
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  2. #2
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    Re: Limits - find parameters γ, δ

    Quote Originally Posted by theintervurt View Post
    Made a new thread for this as instructed by Plato. On to the limit itself

    \lim _{x\mapsto 0} \frac {\gamma + \delta \sin {x}}{x^2} = 3

    I found this in a book with exercises and although Plato mentioned it has no solutions the book says they're γ = 6 and δ = -6 . Also I double checked it and there is no x in front of γ .
    That practice book is completely wrong.
    There is no solution to that limit as written.

    It must be a typo in that text.
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  3. #3
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    Re: Limits - find parameters γ, δ

    Forgive me I just realized it's not a sine but a cosine up there, I learn everything by reading greek books and the english equivalents and terminology is new to me so I might get some of it wrong.
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  4. #4
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    Re: Limits - find parameters γ, δ

    Hello, theintervurt!


    We need these two facts:

    . . 1 - \cos\theta \:=\:2\sin^2\tfrac{\theta}{2}

    . . \lim_{\theta\to0}\frac{\sin\theta}{\theta} \:=\:1



    \lim _{x\to 0} \frac {a + b\cos x}{x^2} \:=\: 3

    \text{We have: }\:f(x) \:=\:\frac {a + b\cos x}{x^2}

    \text{Factor out }a\!:\;\;f(x) \:=\: \frac{a\left(1 + \frac{b}{a}\cos x\right)}{x^2}{

    \text{Let }\tfrac{b}{a} = \text{-}1\!:\;f(x) \:=\:\frac{a(1 - \cos x)}{x^2}

    \text{Then: }\:f(x) \:=\:\frac{a\cdot 2\sin^2\frac{x}{2}}{x^2} \:=\:\frac{\frac{1}{4}\left(2a\sin^2\frac{x}{2} \right)}{\frac{1}{4}(x^2)} \:=\:\tfrac{1}{2}{a}\frac{\sin^2\frac{x}{2}}{( \frac{x}{2})^2} \:=\:\frac{a}{2}\left(\frac{\sin\frac{x}{2}}{ \frac{x}{2}}\right)^2


    \text{Since }\lim_{x\to0} f(x) \:=\:3,\,\text{ we have:}

    . . \lim_{x\to0}\frac{a}{2}\left(\frac{\sin\frac{x}{2}  }{\frac{x}{2}}\right)^2 \:=\:3 \quad\Rightarrow\quad \frac{a}{2}(1^2) \:=\:3 \quad\Rightarrow\quad \frac{a}{2} \:=\:3


    \text{Therefore: }\:a = 6,\;b = \text{-}6
    Thanks from theintervurt
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