What is sigma notation for a geometric series with first term `a` and common ratio `r` ?

Related questions

• How does sigma notation work?
• How do you use sigma notation to represent the series `1/2+1/4+1/8+…`?
• Use summation notation to express the sum?
• What is sigma notation for an arithmetic series with first term `a` and common difference `d` ?
• How do you evaluate the sum represented by `sum_(n=1)^5n/(2n+1)` ?
• How do you evaluate the sum represented by `sum_(n=1)^(8)1/(n+1)` ?
• How do you evaluate the sum represented by `sum_(n=1)^(10)n^2` ?
• What is the value of `1/n sum_{k=1}^n e^{k/n}` ?
• Question #07873
• Question #117a3