Question

write a formula for the geometric sequence 5,10 20,40,....

Answer 1

Do you know the formula for a geometric sequence?
It's \[ar ^{n-1}\]
where a = The first term
r = The common ratio
n = You know n, don't you!?

Answer 2

Here, a = 5
r = 2
and n = n
Just plug in the values.