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Chelsea04

  • 3 years ago

In the expansion of (2a − 1)^n, the coefficient of the second term is −192. Find the value of n.

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  1. Chelsea04
    • 3 years ago
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    I've gotten pretty far for this question but I just need a little help with the rest

  2. Chelsea04
    • 3 years ago
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    \[\left(\begin{matrix}n \\ 1\end{matrix}\right)(2a)^{n-1}(-1)^1=-192a^{n-1}\]

  3. Chelsea04
    • 3 years ago
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    \[-(\frac{ n! }{ (n-1)!1! })(2a)^{n-1}=-192a^{n-1}\] \[-\frac{ n! }{ (n-1)! }=-\frac{ 192a^{n-1} }{ 2a^{n-1} }\] \[\frac{ n! }{ (n-1)! }=192\]

  4. Chelsea04
    • 3 years ago
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    from here I do not know what to do

  5. hartnn
    • 3 years ago
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    \(\large (2a)^{n-1}= 2^{n-1}a^{n-1}\)

  6. Chelsea04
    • 3 years ago
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    oh, oops!

  7. hartnn
    • 3 years ago
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    also, \(n! = n (n-1)!\)

  8. Chelsea04
    • 3 years ago
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    oh, right I think I can solve the rest by myself. I'll ask you if I need any more help... THANK YOU SOOO SOOO MUCH!!!

  9. hartnn
    • 3 years ago
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    WELCOME SOOO SOOO MUCH!!! ^_^

  10. Chelsea04
    • 3 years ago
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    I am up to here:\[2^{n-1}=\frac{ 192 }{ n }\]and I don't know what to do... Someone please help!

  11. klimenkov
    • 3 years ago
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    You can just put \(n=1,2,3,\ldots\) until you get the equality.

  12. Chelsea04
    • 3 years ago
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    I know what the answer is, but I need to know how to get there

  13. klimenkov
    • 3 years ago
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    It cant be solved so easy. The way to solve is just to put the values of n into the equation and watch what you get.

  14. Chelsea04
    • 3 years ago
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    now I am up to here: \[2^n=\frac{ 384 }{ n }\]

  15. klimenkov
    • 3 years ago
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    This is a transcendential equation. There is no method to solve it using elementary operations and functions.

  16. sami-21
    • 3 years ago
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    well using newton method i am getting n=5.999 are you familiar with newton method ???

  17. Chelsea04
    • 3 years ago
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    no, but I know that the answer is 6

  18. sami-21
    • 3 years ago
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    ok newton method is a numerical technique used to solve equations . you familiar with derivatives ?

  19. Chelsea04
    • 3 years ago
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    a little, I mean I am only an 11th grader

  20. sami-21
    • 3 years ago
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    hmm then i would say go for hit and trail .. start with n=0 ,1,2... untill you get the result .

  21. Chelsea04
    • 3 years ago
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    sure, thank you anyways for your help :)

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