How do I use the sigma notation to express the series : 1+5+25+125+625+3125?

Answer 1

First try to make a relation/rule between the different terms:
1=5⁰
5=5¹
25=5²
125=5³
625=5⁴
3125=5⁵

So you are really summing
5⁰+5¹+5²+5³+5⁴+5⁵

So you would end up with:

i=5
Σ 5ⁱ
i=0

Answer 2

Thank you helps a lot😁