Can anyone help me solve this?

My answer is 5.

My friend's got 17.

Im confused.

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- Apr 19th 2013, 06:13 PMweijing85Finding Interval of Convergence
Can anyone help me solve this?

My answer is 5.

My friend's got 17.

Im confused. - Apr 19th 2013, 07:10 PMGusbobRe: Finding Interval of Convergence

The series

$\displaystyle \frac{1}{1+z}=1-z+z^2-z^3+z^5-z^5+...$

is convergent if and only if $\displaystyle |z|<1$.

In your case, $\displaystyle z=\frac{x-12}{5}$, so $\displaystyle |x-12|<5$. If $\displaystyle x\geq 12$, then you get $\displaystyle x-12<5 \implies x<17$. If $\displaystyle x\leq 12$, you get $\displaystyle -x+12<5 \implies x> 7$. Thus your interval of convergence is $\displaystyle (7,17)$

So your friend is correct. - Apr 20th 2013, 01:24 AMweijing85Re: Finding Interval of Convergence
thanks! ((: