# Assignment 14, Q2(ii)

How do I find the sum of the geometric progression?
I found this formula: Sigma from k=0 to n of q^k =(1-q^(n+1))/(1-q). But this gives me 3/(2-√3) and how do I establish this formula? Or is this a formula one should generally know?

### Formula booklet

The formula for the sum of a geometric progression is on the formula booklet for STEP: http://www.admissionstestingservice.org/images/121915-step-formula-bookl...

However, it is one that it's nice to have at your fingertips without having to look up, and I always find it easier to remember a formula if I know where it comes from! If you have not seen the derivation before you might want to take a look at this problem: http://nrich.maths.org/8054 and its solutions.

### Assignment 3

You can also look at assignment 3 question 1(ii) where the formula for the sum of a geometric series is derived.

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