Find the radius and disk of convergenct

/in /by

Find the radius and disk of convergenct

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please i need it bu this method

General form of power series is  sum  an (z-center)^n

so  the value of radius =  limit |an| / |an+1 |

n: infinity

then the convergent is  less than the radius

but must note : if the radius is zero  the convergent become point zero  , if infinity is all