MHB Geometric Progression sequence with an Arithmetic Progression grouping problem

AI Thread Summary
The discussion revolves around solving a problem involving a geometric progression sequence grouped by an arithmetic progression. The proposed solution is 2^[(n^2 + n)/2] - 1, but participants are struggling to understand how to derive this from the sum of terms in the specified range. The formula for the sum of a geometric series is introduced, leading to the conclusion that the sum should be computed from the specified indices. Clarification is sought on how to simplify the expression to match the given solution. The conversation highlights the complexity of transitioning between the two summation forms.
nicodemus1
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Good Day,

My friends and I are stuck on solving the last part of the attached problem.

The solution is 2^[(n^2 + n)/2] - 1.

Can anyone help us with solving this?

Thanks & Regards,
Nicodemus
 
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The solution you give would be the sum of all the terms in the first n groups, not the sum of just the terms in the nth group.

Let:

$\displaystyle p<q$ where $\displaystyle p,q\in\mathbb{N}$

and then:

$\displaystyle S=2^p+2^{p+1}+2^{p+2}+\cdots+2^{q}$

$\displaystyle 2S=2^{p+1}+2^{p+2}+2^{p+3}+\cdots+2^{q}+2^{q+1}$

Subtracting the former from the latter, we find:

$\displaystyle S=2^{q+1}-2^p$

Now, let:

$\displaystyle p=\frac{n^2-n}{2},\,q=\frac{n^2+n}{2}-1$
 
Good Day,

Thank you for the reply.

However, I don't see how it simplifies to the given solution. If it does, then I would first have to divide the expression by a term, right? How do I obtain that term and division from?

Thanks & Regards,
Nicodemus
 
The given solution is for:

$\displaystyle \sum_{k=0}^{\frac{n^2+n}{2}-1}2^k$

However, you are being asked to compute:

$\displaystyle \sum_{k=\frac{n^2-n}{2}}^{\frac{n^2+n}{2}-1}2^k$
 
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