# Math Help - arithmetic series

1. ## arithmetic series

calculate the sum of the first 50 terms.
1+9+17+ ...

2. ## Re: arithmetic series

Use $S_n = \frac{n}{2}(2a+(n-1)d)$

n=50 so,

$S_{50} = \frac{50}{2}(2a+(50-1)d)$

now all you need is 'a' the first term and 'd' the common difference from the series, what do you get?

3. ## Re: arithmetic series

thanks.
and whats the geometric series forumla???

5. ## Re: arithmetic series

what does the r represent?

6. ## Re: arithmetic series

Originally Posted by ahmedb
what does the r represent?
r is defined in the article linked by pickslides ... look for it.