#### eddy2017

##### Senior Member

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Greetings Mr Khan, I did check it out with the formula and it checks out. The result is the same.

Sn= a(1-r^2) / 1-r

Sn = 100 ( 1-2^n) / (1-2)

Sn =102300

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- Thread starter eddy2017
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Greetings Mr Khan, I did check it out with the formula and it checks out. The result is the same.

Sn= a(1-r^2) / 1-r

Sn = 100 ( 1-2^n) / (1-2)

Sn =102300

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Yes, Otis, thanks for your help and support. I have bought several book. Algebra and geometry. I'll take some pics and send them to you. They are a great source of guidance like you said. As I explain Mr mmm the moderator( who, by the way, stayed in touch with me while I was banned!), amazing guy who kept wanting to teach me things)) I got my bachelor's degree back in Cuba where i taught ESOL in a teacher's college. I got credentialed when i got here and turned out to have a us bachelor degree so I have the opportunity to take tests to become a teacher without having to incur a humongous debt going to college and in a lot less time studying on my own. Right now I am teaching ESOL at a public school in Miami FL( I was loading and unloading trucks for some time), but I want to become a math teacher , at least, from 5-to 9 math teacher. I have to pass a Pearson test for math teachers. 5 to 9 math test. I t is not that difficult but i am prepping for that non-stop.No, Eddy. You're confusing the geometric sequence with an arithmetic sequence. The terms in anarithmeticsequence differ by the same amount (that's why it's called a commondifference).

With geometric sequences, we call r the commonratiobecause (starting with the second term) the ratio of each term to the prior term is always the same: r. Here are four examples:

800/400 = 2

400/200 = 2

200/100 = 2

a_{n}/ a_{n-1}= 2

I forgot. Are you working with math textbooks now?

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using the formula we got the desired result,

Question: Why couldn't we get the same result doubling the amounts by every week until week number 10, like i tried to do in post 12 i think, why?. It is supposed to work out with addition too, right, or not? or I performed the addition in the wrong way. Just wondering.

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Omitting parentheses is a serious mistake. I am not sure why so many students are so careless about them. The consequences of this mistake are just as bad as omitting a negative sign or any other math error. In this case you put them back on the second line, but next time you'll forget. Parentheses are not a cosmetic thing to make your expressions look pretty. They are absolutely necessary to get the correct result.Greetings Mr Khan, I did check it out with the formula and it checks out. The result is the same.

Sn= a(1-r^2) / 1-r

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Post 12 shows why your addition was incorrect - you didn't calculate all the individual amounts correctly.

using the formula we got the desired result,

Question: Why couldn't we get the same result doubling the amounts by every week until week number 10, like i tried to do in post 12 i think, why?. It is supposed to work out with addition too, right, or not? or I performed the addition in the wrong way. Just wondering.

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Thank you very much, Mr lev. And you are right about parenthesis. I tend to forget them. I will have to keep an eye out for them. I am using post-it notes for things I can't forget to do. I'm starting to do the same with some of the math must-do's.Post 12 shows why your addition was incorrect - you didn't calculate all the individual amounts correctly.

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