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hamid14

Thu Jan 27, 2011 6:35 pm


sequences and series

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it says find the sum of the series :

3 - 6 + 12 - 24 + ... + 3072

I made a formula Sn = 3 * -2 (n-1) and the n-1 is an exponent 
but i cant figure out who to get the sum properly. thanks any help is appreciated.


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Insectoid

Thu Jan 27, 2011 6:42 pm


RE:sequences and series

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Try writing out the first few values to generate a formula.

sum(1) = 3
sum(2) = -3
sum(3) = 9
sum(4) = -15
...

With this information it's relatively straightforward to figure it out.


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hamid14

Thu Jan 27, 2011 6:52 pm


Re: sequences and series

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lmao sorry but im just not getting this. im actually trying to help someone else.


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RandomLetters

Thu Jan 27, 2011 8:13 pm


RE:sequences and series

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It's pretty hard to explain how to find the formula.
Use the basic exponential sequence tn = 2^(n-1)

Then

Sn = 2 + 4 + 8 + 16 ... tn

then some magic happens

And you get the formula Sn = a(1 - r^n)/(1-r)


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hamid14

Thu Jan 27, 2011 9:57 pm


Re: sequences and series

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lol great explanation jk. i got it >.>


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L.mawaty

Sun Jan 30, 2011 10:20 pm


RE:sequences and series

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:)