I've been working on this for 2 days and I'm at the edge of giving up. Can somebody help explaining this question to me :')

[Iseul Stanmore]

 

[Pedja]

Hi. We have:
[math]S_n=a_1\left(\frac{1-r^n}{1-r}\right) \quad \text{for} \quad r \neq 1[/math]and
[math]S_{\infty}=\frac{a_1}{1-r} \quad \text{for} \quad |r|<1[/math]where [imath]a_1[/imath] is the first term.

 

[Iseul Stanmore]

Hi. We have:
[math]S_n=a_1\left(\frac{1-r^n}{1-r}\right) \quad \text{for} \quad r \neq 1[/math]and
[math]S_{\infty}=\frac{a_1}{1-r} \quad \text{for} \quad |r|<1[/math]where [imath]a_1[/imath] is the first term.

thank you so muchhh ???