I need a little help in summing the the following series:
$$ 1+2v^4+3v^8+4v^{12}+\ldots + 20v^{76}?$$
Is there a closed formula for summing $$ \sum_{k=0}^{n} k\cdot ar^k?$$
I need a little help in summing the the following series:
$$ 1+2v^4+3v^8+4v^{12}+\ldots + 20v^{76}?$$
Is there a closed formula for summing $$ \sum_{k=0}^{n} k\cdot ar^k?$$