Evaluating \(\displaystyle{\sum_{{{n}={0}}}^{{{20}}}}{\frac{{{\left(-{1}\right)}^{{{n}}}{2}^{{{n}+{1}}}}}{{{3}^{{{n}}}}}}\)

Marzadri9lyy

Marzadri9lyy

Answered question

2022-04-05

Evaluating
n=020(1)n2n+13n

Answer & Explanation

Ashton Conrad

Ashton Conrad

Beginner2022-04-06Added 11 answers

Define
S=a+ar++arn1
for r1, and multiply by r to get
rS=ar+ar2++arn
Subtracting S from rS gives
rSS=arna
or
S(r1)=arna
Therefore
S=arn1r1

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