Answer:
Radius of the convergence is R = ![\frac{1}{21}](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B21%7D)
and,
The Interval of convergence is ![-\frac{1}{21}](https://tex.z-dn.net/?f=-%5Cfrac%7B1%7D%7B21%7D%3Cx%3C%5Cfrac%7B1%7D%7B21%7D)
Step-by-step explanation:
Given function : Σ(21x)^k
Now,
Using the ratio test, we have
R = ![\lim_{n \to \infty} |\frac{21x^{k+1}}{21x^{k}}|](https://tex.z-dn.net/?f=%5Clim_%7Bn%20%5Cto%20%5Cinfty%7D%20%7C%5Cfrac%7B21x%5E%7Bk%2B1%7D%7D%7B21x%5E%7Bk%7D%7D%7C)
or
R = ![\lim_{n \to \infty} |\frac{21x^{k}\times21x^{1}}{21x^{k}}|](https://tex.z-dn.net/?f=%5Clim_%7Bn%20%5Cto%20%5Cinfty%7D%20%7C%5Cfrac%7B21x%5E%7Bk%7D%5Ctimes21x%5E%7B1%7D%7D%7B21x%5E%7Bk%7D%7D%7C)
or
R = ![\lim_{n \to \infty} |21x^{1}|](https://tex.z-dn.net/?f=%5Clim_%7Bn%20%5Cto%20%5Cinfty%7D%20%7C21x%5E%7B1%7D%7C)
now,
for convergence R|x| < 1
Therefore,
< 1
or
![-\frac{1}{21}](https://tex.z-dn.net/?f=-%5Cfrac%7B1%7D%7B21%7D%3Cx%3C%5Cfrac%7B1%7D%7B21%7D)
and,
Radius of the convergence is R = ![\frac{1}{21}](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B21%7D)
and,
The Interval of convergence is ![-\frac{1}{21}](https://tex.z-dn.net/?f=-%5Cfrac%7B1%7D%7B21%7D%3Cx%3C%5Cfrac%7B1%7D%7B21%7D)
4 one coefficient and three variables
I seen your comment on the other persons thing, it should be 60° :) correct me if i’m wrong!
I think its A once again i THINK its A sorry if im wrong
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