Calculate the sum of the next series and for which values of x it converges: sum_(n=0)^infty (x+2)^(n+2)/3^n

figoveck38

figoveck38

Answered question

2022-11-03

Calculate the sum of the next series and for which values of x it converges:
n = 0 ( x + 2 ) n + 2 3 n

Answer & Explanation

teleriasacr

teleriasacr

Beginner2022-11-04Added 21 answers

For | ( x + 2 ) / 3 | < 1 it converges to the limit given by multiplication of geometric series limit and polynomial:
n = 0 ( x + 2 ) n + 2 3 n = ( x + 2 ) 2 n = 0 ( x + 2 3 ) n = ( x + 2 ) 2 1 1 x + 2 3 = 3 ( x + 2 ) 2 1 x
for | ( x + 2 ) / 3 | 1, the sum is not convergent.
limunom623

limunom623

Beginner2022-11-05Added 3 answers

Your sum is equal to
n = 0 ( x + 2 ) n + 2 3 n = ( x + 2 ) 2 n = 0 ( x + 2 3 ) n ,
which you recognize as a geometric series n q n with sum 1 / ( 1 q ). Thus,
n = 0 ( x + 2 ) n + 2 3 n = ( x + 2 ) 2 1 1 x + 2 3 = 3 ( x + 2 ) 2 1 x
as long as | ( x + 2 ) / 3 | < 1
If | ( x + 2 ) / 3 | 1, the sum is not convergent.

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