A geometric sequence (or progression) is a sequence of non-zero numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio (r).

Key Formulas:

• N-th Term ($a_n$): To find any term in the sequence.
  $a_n = a_1 \cdot r^{(n-1)}$
• Sum of the First n Terms ($S_n$): To find the sum of a part of the sequence.
  $S_n = a_1 \frac{1 - r^n}{1 - r}$    (for $r \neq 1$)

This calculator can solve for any of the variables ($a_n, S_n, a_1, r, n$) if you provide the others.