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Trig limit question

  1. Jun 29, 2007 #1

    dnt

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    1. The problem statement, all variables and given/known data

    evaluate the limit as x goes to 0 of:

    tan(4x^2)^2/(3x^4)

    (thats suppose to say the tangent squared of 4 x squared all over 3 x to the 4th)

    2. Relevant equations

    n/a

    3. The attempt at a solution

    the only trig limit stuff i know is the limit to 0 of sinx / x = 1

    and

    limit to 0 of (1-cos x)/x = 0

    is there a tangant one i should know? what about tangant squared? i also considered changing it to sin squared over cos squared but i dont know what i could do that with.

    if anyone could give me a hint to get me started, i would appreciate it. thanks.
     
  2. jcsd
  3. Jun 29, 2007 #2

    dnt

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    hmmm actually i think i have an idea (strange how just typing it out can make your mind think in different ways).

    after changing it to sin^2 over cos^2 i can then rewrite it like this:

    (1/3)(sin4x^2/x^2)(sin4x^2/x^2)(1/cos4x^2)(1/cos4x^2)

    then multiply the two sin parts by 4/4 which if i then apply the limit will give me:

    (1/3)(4/1)(4/1)(1/1)(1/1)

    which is 16/3

    is that correct?
     
  4. Jun 29, 2007 #3

    berkeman

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    Staff: Mentor

    In LaTex:

    [tex]\frac{tan^2 (4x^2)}{3x^4}[/tex]

    or do you mean:

    [tex]\frac{tan^2 (4x)^2}{(3x)^4}[/tex]

    Or something else? If you quote my reply, you will see the format of the LaTex inside... It would help to see your equations in LaTex to eliminate the confusion. There's also a new LaTex editing feature in the reply dialog -- in the upper right corner of the entry box, there is a [tex]\Sigma[/tex] symbol to click to get the LaTex editor.
     
    Last edited: Jun 29, 2007
  5. Jun 29, 2007 #4

    dnt

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    its this one:

    [tex]\frac{tan^2 (4x^2)}{3x^4}[/tex]
     
  6. Jun 29, 2007 #5

    berkeman

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    Staff: Mentor

    Okay. Now can you show your steps to the solution using LaTex to make them easier to check? Thanks.

    BTW, the LaTex preview feature now works here on the PF (as of a day or two ago). When you're in the Advanced Reply window, just click on "Preview Post" to make sure you're happy with the appearance of the LaTex.
     
  7. Jun 29, 2007 #6

    Dick

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    Science Advisor
    Homework Helper

    You've got two ways to go here. You can blindly apply L'Hopital's rule through 4 differentiations or you can look at an expansion of tan(x) for small x and go more or less directly to the answer.
     
  8. Jun 30, 2007 #7
    Or you can write it as [tex](\frac{tan4x^2}{4x^2})^2\frac{16}{3}[/tex]. Can you find the limit now?
     
    Last edited: Jun 30, 2007
  9. Jun 30, 2007 #8

    VietDao29

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    Homework Helper

    Yes, that's correct. :smile:

    When seeing tangent function, you can just split it into sin, and cos function, and simply go from there.
     
  10. Jun 30, 2007 #9

    dnt

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    interesting. since i know the answer is 16/3, can i assume the limit of [tex](\frac{tan4x^2}{4x^2})^2\[/tex] when approaching 0 is 1? ive never heard of that limit rule for tangent before.
     
  11. Jun 30, 2007 #10

    VietDao29

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    Homework Helper

    Well, actually, you can derive it from the well-known limit:
    [tex]\lim_{x \rightarrow 0} \frac{\sin x}{x} = 1[/tex]

    Just split tan into sin, and cos, like this:
    [tex]\lim_{x \rightarrow 0} \frac{\tan x}{x} = \lim_{x \rightarrow 0} \frac{\sin x}{x} \frac{1}{\cos x} = 1 \times \frac{1}{1} = 1[/tex]
     
    Last edited: Jun 30, 2007
  12. Jul 1, 2007 #11
    Yeah... thats it.
     
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