# Maths

Find the sum of the following series:
1024 - 512 + 256 - 128 +...+ 1

So a = 1024, r = -0.5, but I need to know what n is in order to use the formula.
So I tried ar^(n-1) which is:
1024(-0.5)^n-1 = 1
but I don't know how to find n from there.
Thanks in advance for any help :)

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1. S = ar * (1 - r^n) / (1 - r)
what is n?
well, take a term 1, double it until you get to 1024
1*2^n=1024
1,2,4,8,16,32,64,128,256,512,1024

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2. Thanks
Although that method works for this question, is there another method that I could use if the method you have used above takes too long if n is a bigger number?

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posted by Rebecca

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