Wednesday 15 February 2017

convergence divergence - Why does the absolute value of a power series converge to the same limit as the power series?



Power series, if $$\lim_{{k}\to\infty}ka_kx^{k-1}=0$$ for any $x\in(-1,1)$



Why does this imply $$\lim_{{k}\to\infty}|ka_kx^{k-1}|=0$$ for any $x\in(-1,1)$




I would normally show what work I've done to try and solve for myself but in this case I just don't know what to say. It seems obvious that if the limit goes to zero then the absolute value would also go to zero but I don't know how to formally state this. Is there something deeper that I'm not seeing, why would this be useful/important?



Any help would be greatly appreciated.



Thank you,


Answer



$$x=0\implies \vert x\vert=0$$
By definition of $\vert x\vert$.


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