Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Possible Duplicate:
sum of a geometric series

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

enter image description here

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.

share|improve this question
    
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
add comment

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

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1 Answer

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.

share|improve this answer
    
I have used the following and get a) 1 b) 1 d) 20 remembering the parentheses. What would the formula be when the upper limit is infinity? is it 1 / (1 - r) –  cocacola09 Feb 8 '11 at 15:22
    
when I use the following formula 1 / (1 - r) I get the following a) -5 b) 0.769230769 c) 28 any help further would be very much appriciated. –  cocacola09 Feb 8 '11 at 15:28
    
And when is 1/(1-r) valid? Not for a. For b, it would be better to leave the result as a fraction, but you are right. c is correct. –  Ross Millikan Feb 8 '11 at 16:10
    
Thank you for your help! it's very much appricated. by the way I used the formula s=(1−rn)/(1−r) for the vulgar fraction. Thanks again! –  cocacola09 Feb 8 '11 at 16:22
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.