Part A
A geometric sequence is where the terms increase by the same ratio.
Example:
7, 14, 28, 56, ...
We start at 7 and double each term to get the next term. The common ratio is 2.
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Part B
The next step is to subtract the two equations straight down. This will cancel the vast majority of the terms, and allow to solve for [tex]S_n[/tex] to get a fairly tidy formula. Refer to part C for more info.
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Part C
[tex]S_n = a_1 + a_1r + a_1r^2 + \ldots + a_1r^{n-1}\\\\rS_n = a_1r + a_1r^2 + \ldots + a_1r^{n-1} + a_1r^n\\\\rS_n - S_n = \left(a_1r + a_1r^2 + \ldots + a_1r^{n-1}+a_1r^n\right)-\left(a_1 + a_1r + a_1r^2 + \ldots + a_1r^{n-1}\right)\\\\S_n(r - 1) = a_1r^n - a_1\\\\S_n = \frac{a_1r^n - a_1}{r-1}\\\\S_n = \frac{-a_1(r^n - 1)}{-(-r+1)}\\\\S_n = \frac{a_1(1-r^n)}{1-r}\\\\[/tex]
For more information about the canceling going on from step 3 to step 4, see the attachment below.
