1. ## Summation

Express $\sum_{n = 1}^N(\frac{1}{8(3^{-2n})})$ in terms of $N$.

$\sum_{n = 1}^N(\frac{1}{8(3^{-2n})}) = (\frac{1}{(8(3^{-2})})+(\frac{1}{(8(3^{-4})})+...+(\frac{1}{(8(3^{-2N})})$

$=\frac{1\frac{1}{8}(1-(\frac{1}{9})^N)}{1-\frac{1}{9}}$

$=\frac{1\frac{1}{8}(1-(\frac{1}{9})^N)}{\frac{8}{9}}$

$=1\frac{17}{64}(1-(\frac{1}{9})^N)$

but ans is $\frac{9}{64}(9^N-1)$

2. I would write $\displaystyle \sum_{n = 1}^N{\frac{1}{8\left(3^{-2n}\right)}} = \frac{1}{8}\sum_{n = 1}^N{9^n}$.

This is a geometric series with $\displaystyle a = 9, r = 9, n = N$. Now apply $\displaystyle S_n = \frac{a(r^n - 1)}{r - 1}$.

3. Originally Posted by Punch
Express $\sum_{n = 1}^N(\frac{1}{8(3^{-2n})})$ in terms of $N$.

$\sum_{n = 1}^N(\frac{1}{8(3^{-2n})}) = (\frac{1}{(8(3^{-2})})+(\frac{1}{(8(3^{-4})})+...+(\frac{1}{(8(3^{-2N})})$

$=\frac{1\frac{1}{8}(1-(\frac{1}{9})^N)}{1-\frac{1}{9}}$

$\sum_{n=1}^{N} \frac{1}{8\ 3^{-2n}} = \frac{1}{8}$

$=\frac{1\frac{1}{8}(1-(\frac{1}{9})^N)}{\frac{8}{9}}$

$=1\frac{17}{64}(1-(\frac{1}{9})^N)$

but ans is $\frac{9}{64}(9^N-1)$
$\sum_{n=1}^{N} \frac{1}{8\ 3^{-2 n}}= \frac{1}{8}\ \sum_{n=1}^{N} 3^{2n} = \frac{9}{8}\ \frac{1-9^{N}}{1-9} = \frac{9}{64}\ (9^{n}-1)$

Kind regards

$\chi$ $\sigma$

4. Originally Posted by Punch
Express $\sum_{n = 1}^N(\frac{1}{8(3^{-2n})})$ in terms of $N$. but ans is $\frac{9}{64}(9^N-1)$
Write $\sum_{n = 1}^N(\frac{1}{8(3^{-2n})})=\frac{1}{8}\sum_{n = 1}^N(9^N)$

If $S=9+9^2+\cdots+9^N$ then $9S=9^2+9^3+\cdots+9^{N+1}$.

Subtract $8S=9^{N+1}-9=9(9^N-1})$

Can you finish?

5. Originally Posted by Plato
Write $\sum_{n = 1}^N(\frac{1}{8(3^{-2n})})=\frac{1}{8}\sum_{n = 1}^N(9^N)$

If $S=9+9^2+\cdots+9^N$ then $9S=9^2+9^3+\cdots+9^{N+1}$.

Subtract $8S=9^{N+1}-9=9(9^N-1})$

Can you finish?
$S=\frac{9(9^N-1)}{8}$

But this answer varies from the answer given in post 3 and my answer sheet. What you said made sense to me but could there be a mistake in the workings?

6. Originally Posted by Punch
$S=\frac{9(9^N-1)}{8}$
But this answer varies from the answer given in post 3 and my answer sheet. What you said made sense to me but could there be a mistake in the workings?
You forgot part of it: $\frac{1}{8}S=\frac{9(9^N-1)}{64}$

7. thanks!