Geometric sequence question in IB HL mathematics paper 1 november 2010

1. Apr 13, 2013

bajoriay

1. The problem statement, all variables and given/known data

The sum S n of the first n terms of a geometric sequence whose nth term is u n is given by

7n-an / 7n

Where a > 0

Find an expression for un

Find the first term and the common ratio of the sequence

Consider the sum to infinity of the sequence

Determine the values of a such that the sum to infinity exists
Find the sum to infinity when it exists

I tried to make it equal to the equation for Sn but it doesn't seem to be helping.

2. Apr 13, 2013

Ray Vickson

Do you mean
$$u_n = \frac{7^n - a^n}{7^n},$$
of do you mean
$$u_n = 7^n - \frac{a^n}{7^n}?$$
Use parentheses, like this: (7^n - a^n)/7^n or 7^n - (a^n/7^n).

3. Apr 13, 2013

haruspex

That's the right way. Pls post your working.