# Sum series questions

1. Mar 27, 2008

### sweetsurf3

infin
1.) Suppose that (6x)/(10+x)= sum C_n(X^(n))
n=o to

Find: (what I got, but was wrong)
C_0= 0
C_1= 6/11
C_2= 1
C_3= 18/13
C_4= 24/14
Find: Radius of convergence R, I got 10.

I've been struggling with setting any sum problem up.

2) Find the interval of convergence for the given power series: n=1
(x-5)^(n)/n(-10)^(n)
The series is convergent from x=___, left end included (Y,N)___
to x=___, right end included (Y,N)___

any info and pointers is GREATLY appreciated, thank you :)

2. Mar 27, 2008

### HallsofIvy

Staff Emeritus
I would notice that 6/(10+ x)= 0.6(1/(1- x/10)) and recognise that as the sum of a geometric sequence: $\Sum_{i= 0}^{\infty} 0.6(x/10)^i$- which converges as long as |x/10|< 1 or |x|< 10.

Of course, your sum is 6x/(1+ x). Okay, multiply the geometric sum by x: $\Sum{i=0}^{\infty}0.6 x^{i+1}/10^i$. That won't affect the radius of convergence.

To determine whether the series converges at each of the end points, you might want to see whether the series is "alternating" or not.