Originally Posted by

**0123** I know these are very very basic series(if they are..

) but I have no idea of what they actually are, I found them in my way and need to understand why the result is that.

If I have the ? [from 1 to 6] of (1/(1.3)^ (s/12)) how do I solve it and why?

Thanks for all the help.

This is a finite geometric series:

Now you could look up the formula for the sum of a finite geometric series or

derive it form the observation that:

since if you multiply out the brackets on the left all but the first and last

terms in the result cancel out.

So we have:

or:

for any and , and so in particular for our

series, where , and .

Hence:

Reality check:

Code:

>
>.. bute force sum of series:
>
>k=1:6;
>S=(1/1.3)^(k/12);
>SS=sum(S)
5.56187
>
>
>.. CaptainBlack's formula for sum of series:
>
>S6 = (1-1/1.3^(7/12)) / (1-1/1.3^(1/12))-1
5.56187
>

RonL