Find the rth term of this sequence

[Harmony]

Given a sequence:
3,33,333,3333,...
Find the rth term of this sequence.

This sequence does not belong to Geometry Progression or Arithmetic Progression. From what I know, rth term is equals to (r-1)*10 +3.

But how do I solve this problem?

 

[bomba923]

This sequence does not belong to Geometry Progression or Arithmetic Progression. From what I know, rth term is equals to (r-1)*10 +3.

Hint:
\sum\limits_{k = 0}^n {r^k } = \frac{{1 - r^{n + 1} }}{{1 - r}}

The r^th term of your sequence is just (10^r-1)/3