What is a rule for the series below?
1/5 + 1/10 + 1/15 + 1/20 + 1/25
I think it is 1/5k, but the answer key says 1/5^k?
As long as you mean
Tk=1/(5k) is the rule for term k of the given sequence Tk, you're correct.
Perhaps 1/(5^k) refers to a different series.
