Maclaurinrows - Help me understand d)

Translation:
c) Set up a Maclaurin row
d) Use the result in c) to decide the sum of this row

Please help me understand how you find x = 1/3
So far, I think we have to "guess" it, but I suspect it is wrong to guess, and there is a method to find x.

My work:
http://prntscr.com/3plbi2

Question

Maclaurinrows - Help me understand d) 

Translation:
c) Set up a Maclaurin row
d) Use the result in c) to decide the sum of this row

Please help me understand how you find x = 1/3
So far, I think we have to "guess" it, but I suspect it is wrong to guess, and there is a method to find x.

My work:
http://prntscr.com/3plbi2

anonymous · Answer

"FS" in my work means "Formula". I have a little book that I am allowed to use during exam where the formulas for Maclaurinrows are available to me.

loser66 · Answer

@Ivanskodje  still need help?

anonymous · Answer

I do not recall getting help to begin with...
I want to understand D.

loser66 · Answer

sure!!

loser66 · Answer

to find x , you have to consider whether the sum is converge or not, How? by ratio test

loser66 · Answer

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