Geometric Equation Help

A geometric series has first term 729 and common ratio -1/3
Calculate the value of n for which Sn = 1 + Sn-1

How do I go about starting this?

but how do i find n where the difference is just 1 between sn and sn-1

but how do i find n where the difference is just 1 between sn and sn-1

Work out expressions for Sn and Sn-1 and solve for n, surely?

Edit: although this is the obvious thing to do and will work, considering the difference between Sn and Sn-1 is going to be less work.

It is that right?

Okay so here's my logic with your help:

sn-1 = 20 (for this example)
sn therefore needs to = 21
the nth term added to sn-1 must be = 1

so un = 729 x 1/3^(n-1)
1 = 729 x 1/3^(n-1)
ln(1/729) / ln(1/3) = n-1
6 = n-1
7 = n

is that right?

Logic looks fine, but I thought r = -1/3 in the question. Have you lost a minus sign?

i couldnt ln(-1/3)