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Math Help - Somebody check if this is ok!

  1. #1
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    Somebody check if this is ok!

    \lim_{x\rightarrow0}\frac{-42(ln(1+x))+42x}{x^2(e^x+4)}=

    \frac{-42[x-\frac{x^2}{2}+\frac{x^3}{3}...]+42x}{x^2[1+x+\frac{x^2}{2!}+\frac{x^3}{3!}+4]}=

    \frac{[21x^2-14x^3...]}{[5x^2+x^3+\frac{x^4}{2!}...]}

    Division with x^2 gives

    \frac{21-14x...}{5+x+\frac{x^2}{2!}...}= and when  x\rightarrow0 we get

    \frac{21-0}{5+0+0}=\frac{21}{5}??
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  2. #2
    Super Member flyingsquirrel's Avatar
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    Hi

    That's it. By the way notice that the terms x^3,\,x^4,\ldots are useless here :

    \frac{-42\ln(1+x)-42x}{x^2(\exp x+4)}=\frac{-42\left(x-\frac{x^2}{2}+o(x^2)\right)+42x}{x^2(1+o(1)+4)}=\f  rac{21x^2+o(x^2)}{5x^2+o(x^2)} \to \frac{21}{5} as x\to 0
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  3. #3
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    1) You're missing some ellipses in that horrible 2nd denominator.

    2) You physically substituted x = 0 in the last step, even though you divided by x^2 a few steps earlier. This should at least make you feel a little dirty.

    3) I'm not particularly fond of the 3rd numerator. All the shown coefficients are integers. Very deceptive.

    4) You seem to have the idea. Good work.

    5) Can you think of another way to do it? This is useful for checking things for yourself.
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  4. #4
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    thanx

    Quote Originally Posted by flyingsquirrel View Post
    Hi

    That's it. By the way notice that the terms x^3,\,x^4,\ldots are useless here :

    Thanks for your check. When do they become useless. After or before the division with x^2
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  5. #5
    Super Member flyingsquirrel's Avatar
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    Quote Originally Posted by lynch-mob View Post
    Thanks for your check. When do they become useless. After or before the division with x^2
    Before the division : When you divide by x^2 you should see that these "useless" terms tend to 0 so they do not give us any information on the limit, they shouldn't appear at all. To find which terms have to be written, one can simply try several approximations starting from the one which has the lowest order :

    Using \ln x =x+o(x) gives -42\ln (1+x)+42x=o(x) which doesn't give us any information on the limit since it is divided by x^2(\exp x+4)

    Using \ln x =x-\frac{x^2}{2}+o(x^2) gives -42\ln (1+x)+42x=-\frac{x^2}{2}+o(x^2) which is enough to get the limit since one can give an approximation of the denominator with order two.
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  6. #6
    MHF Contributor Mathstud28's Avatar
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    Quote Originally Posted by lynch-mob View Post
    \lim_{x\rightarrow0}\frac{-42(ln(1+x))+42x}{x^2(e^x+4)}=

    \frac{-42[x-\frac{x^2}{2}+\frac{x^3}{3}...]+42x}{x^2[1+x+\frac{x^2}{2!}+\frac{x^3}{3!}+4]}=

    \frac{[21x^2-14x^3...]}{[5x^2+x^3+\frac{x^4}{2!}...]}

    Division with x^2 gives

    \frac{21-14x...}{5+x+\frac{x^2}{2!}...}= and when  x\rightarrow0 we get

    \frac{21-0}{5+0+0}=\frac{21}{5}??
    A slight trick you could have done would be

    e^x+4=e^x-1+5\sim{x+5}
    It could have saved some writing
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