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limit with sums

Vali

New member
Hi!


I have the following sequence \(\displaystyle (x_{n})_{n\geq 1}, \ x_{n}=ac+(a+ab)c^{2}+...+(a+ab+...+ab^{n})c^{n+1}\)
Also I know that \(\displaystyle a,b,c\in \mathbb{R}\) and \(\displaystyle |c|<1,\ b\neq 1, \ |bc|<1\)
I need to find the limit of \(\displaystyle x_{n}\).


My attempt is in the picture.The result should be \(\displaystyle \frac{ac}{(1-bc)(1-c)}\)
I miss something at these two sums which are geometric progressions.Each sum should start with \(\displaystyle 1\) but why ? If k starts from 0 results the first terms are \(\displaystyle bc\) and \(\displaystyle c\) right?

my.jpg
 

Vali

New member
I'm stupid, I got the correct answer.I just needed to solve some little calculations.I don't know I thought I'm wrong..
Thanks!
 
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