Maths Core 2 Sequences Q!

Hello Can anyone explain to me what the "sum to infinity of a series" is and how to work it out?? Thank you

If you want a proper definition, the sum to infinity of the sequence where the ith term is defined by $u_i$ is $S_{\infty} =\displaystyle\lim_{n\rightarrow \infty} \displaystyle\sum_{i=1}^{n} u_i$

In C2, these will be geometric series with a first term a and common ratio r, where |r| < 1.
You just use the equation in the formula book.
$S_{\infty} = \frac{a}{1-r}$