CPM Homework Help : CALC Problem 9-137

For each of the series below, write an equivalent expression using sigma notation.

$5 + 10 + 20 + 40 + ... + 5(2)^{n-1}$
Answer (a):
$\displaystyle\sum_{i=1}^{n}5(2)^{i-1}$

$1 + \frac { 2 } { 3 } + \frac { 4 } { 9 } + \frac { 8 } { 27 } + \dots$
Step (b):
$a_n=\Big(\frac{2}{3}\Big)^n\text{ starting with }n=0.$