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Calculus Level 4

Let

S = 3 2 2 × 1 × 2 + 4 2 3 × 2 × 3 + 5 2 4 × 3 × 4 + 6 2 5 × 4 × 5 + \displaystyle S = \frac{3}{2^{2}\times 1 \times 2} + \frac{4}{2^{3}\times 2 \times3} + \frac{5}{2^{4}\times 3 \times 4} + \frac{6}{2^{5} \times 4 \times 5} + \ldots

Then the value of

S 12 \displaystyle S^{-12} is ?

Note

  • Try to find the n t h n^{th} term of the series

  • Happy Diwali to all


The answer is 4096.

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1 solution

Krishna Sharma
Oct 23, 2014

S = r = 1 r + 2 2 r + 1 . r ( r + 1 ) \displaystyle S = \sum_{r = 1}^{\infty } \frac{ r + 2}{2^{r+1}.r(r + 1)}

= r = 1 2 ( r + 1 ) r 2 r + 1 . r ( r + 1 ) \displaystyle = \sum_{r = 1}^{\infty} \frac{2(r+1) -r}{2^{r+1}.r(r+1)}

Now seperating

r = 1 1 2 r + 1 ( 2 r 1 r + 1 ) \displaystyle \sum_{r=1}^{\infty } \frac{1}{2^{r + 1}}( \frac{2}{r} - \frac{1}{r + 1})

= r = 1 ( 1 2 r . r 1 2 r + 1 . ( r + 1 ) ) \displaystyle = \sum_{r = 1}^{\infty} ( \frac{1}{2^{r}.r} - \frac{1}{2^{r+1}.(r+1)})

All the terms will cancel out as we substitute values of 'r' (first term will remain as it is)

Therefore we finally get

S = 1 2 \frac{1}{2}

Hence

S 12 = 4096 \displaystyle S^{-12} = \boxed{4096}

It took very long time to think about the general term

nice question

but why the calculus tag?

U Z - 6 years, 7 months ago

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Series and sequences are under calculus but ok I changed

Krishna Sharma - 6 years, 7 months ago

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I think it would help to add maybe one or two more terms to the sequence although it's not necessary

Trevor Arashiro - 6 years, 7 months ago

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@Trevor Arashiro 4 terms are fine

Krishna Sharma - 6 years, 7 months ago

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@Krishna Sharma I agree with Trevor that the sequence is not obvious, and that you should list out the pattern of the terms instead of obscuring it.

I have edited the problem to make it easier to understand what you are asking for.

Calvin Lin Staff - 6 years, 7 months ago

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@Calvin Lin Main objective of the problem was to find the n t h n^{th} term of the sequence, you should have added one more term instead of simplifing.

P.S - I have already mentioned in the problem 'try to find n t h n^{th} term of the series'

Krishna Sharma - 6 years, 7 months ago

@Calvin Lin Sir now it becomes much simpler

U Z - 6 years, 7 months ago

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