A geometic series of n terms that can be written:

a + ar + ar^2 + ar^3 + ... + ar^n-1 where a is the first term and r is the common ratio. If I express this in sigma notation would the lower limit be k=1 and the upper limit be n and the sum be r^n-1? and how would I write a forumula for this? Could I write Sn= a/(1-r) when -1 < r < +1?