Let
S = 2 2 × 1 × 2 3 + 2 3 × 2 × 3 4 + 2 4 × 3 × 4 5 + 2 5 × 4 × 5 6 + …
Then the value of
S − 1 2 is ?
Note
Try to find the n t h term of the series
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It took very long time to think about the general term
nice question
but why the calculus tag?
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Series and sequences are under calculus but ok I changed
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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
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@Trevor Arashiro – 4 terms are fine
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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.
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@Calvin Lin – Main objective of the problem was to find the n t h 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 term of the series'
@Calvin Lin – Sir now it becomes much simpler
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S = r = 1 ∑ ∞ 2 r + 1 . r ( r + 1 ) r + 2
= r = 1 ∑ ∞ 2 r + 1 . r ( r + 1 ) 2 ( r + 1 ) − r
Now seperating
r = 1 ∑ ∞ 2 r + 1 1 ( r 2 − r + 1 1 )
= r = 1 ∑ ∞ ( 2 r . r 1 − 2 r + 1 . ( r + 1 ) 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 = 2 1
Hence
S − 1 2 = 4 0 9 6