# Math Help - Summation of summation?

1. ## Summation of summation?

I want to figure out the proper summation to figure out a sum of products:
5
x3
----
15
x3
----
45
x3
----
135
x3
___
...

So if n=0 the result is 5, if n=1 the result is 5+15, if n=2, the result is 5+15+45 and so. Hopefully that makes sense. I'm not quite sure how else to put it. Would the best way be to do something like $\sum_{i=0}^{n}5\sum_{j=0}^{i}3^j$? Or is there some more concise way of putting it? I appreciate any help you guys can offer. Thanks.

I want to figure out the proper summation to figure out a sum of products:
5
x3
----
15
x3
----
45
x3
----
135
x3
___
...

So if n=0 the result is 5, if n=1 the result is 5+15, if n=2, the result is 5+15+45 and so. Hopefully that makes sense. I'm not quite sure how else to put it. Would the best way be to do something like $\sum_{i=0}^{n}5\sum_{j=0}^{i}3^j$? Or is there some more concise way of putting it? I appreciate any help you guys can offer. Thanks.
Try

$\displaystyle 5\sum_{i=0}^n3^i$