# sum of a geometric series

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

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

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

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

using the formula

s= 1 - r^n / 1 - r

1)

1 - 6/5 = -0.2

1 - -0.2 = 1.2 or 6/5

2)

1 - -3/10 = 1.3

1 - 1.3 = -0.3 = -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.

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By the way why was this voted done? –  cocacola09 Feb 8 '11 at 14:14
I have reasked my question with answers provided. math.stackexchange.com/questions/20999/… –  cocacola09 Feb 8 '11 at 14:15
@Akhil: Why did you add the homework tag? Did cocacola09 say it actually was homework? –  Isaac Feb 8 '11 at 14:39
@Isaac: As you may have noticed, he now has said that it is homework at the duplicate question. (This was after your comment, let alone Akhil's edit, and I am not questioning your question.) –  Jonas Meyer Feb 8 '11 at 17:27