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CALLING MATHS GENIUSES! series & sequences - sigma notation question?
hello everyone,
i'm doing IB maths SL and i'm stuck on a question (Q9, Ex 6E in case any of you IB-ers out there are reading this).
It is:
Given that [infinity (over) sigma (over) r=0] 5*a^r = 15, find the value of a.
I hope my equation format wasn't too confusing 
Thank youu!
Its a geometric series with an intial value of 5, and an unkown ratio = a.
The formula for the sum of an infinite geometric series is:
(initial value)/(1-ratio) = 5/(1-a) = 15
So 1-a = 1/3.
Hence a = 2/3.
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