# infinity geometric series [duplicate]

Possible Duplicate:
sum of a geometric series

I have the following questions, answers are below, can anyone tell me if the following is correct.

using the formula

s= 1 - r^n / 1 - r

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

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.

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You should be able to edit your original question instead of adding a new one. Please go do that, including showing your work. All of these answers are wrong, besides misusing the equals sign. –  Ross Millikan Feb 8 '11 at 14:29
Hi Ross, can you tell my why they are incorrect please? –  cocacola09 Feb 8 '11 at 14:40
I have now included the formula along with the working out for each. –  cocacola09 Feb 8 '11 at 14:49
It's my homework I hope im correct lol :( –  cocacola09 Feb 8 '11 at 14:52

## marked as duplicate by Ross Millikan, Isaac, Arturo Magidin, Alex B., Sivaram Feb 8 '11 at 15:39

Your formula s= 1 - r^n / 1 - r is incorrect. It should be $s= (1 - r^n )/( 1 - r)$ and that is when the upper limit is $n$, not $\infty$ Parentheses matter. You can never have two answers to a question like this, so 6/5 or infinity cannot be correct. Have another go.