sum of a geometric series

Question

Hey, I have a couple of questions, I was hoping someone could help me with?

\begin{equation} \displaystyle\sum_{n=0}^{\infty} (- \frac{6}{5})^n \end{equation}

\begin{equation} \displaystyle\sum_{n=0}^{\infty} (- \frac{3}{10})^n \end{equation}

\begin{equation} \displaystyle\sum_{n=0}^{\infty} 20(\frac{2}{7})^n \end{equation}

using the formula

s= 1 - r^n / 1 - r

answers

1)

1 - 6/5 = -0.2

1 - -0.2 = 1.2 or 6/5

Answer: 6/5 or infinity

2)

1 - -3/10 = 1.3

1 - 1.3 = -0.3 = -3/10

Answer: -3/10

3)

1 - 2/7 = 0.714285714

1 - 0.714285714 = 0.285714286 or 5 2/7

5 2/7 or infinity

the a/b denotes a fraction.

Thank you in advance.

Answer 1

This would appear to be homework-please tag as such. Do you know the usual formula for the sum of a geometric series? And when it applies? These all fit in nicely.