Geometric Progression

Formulas:

• The common ratio:
  $r = \dfrac{a_2}{a_1} = \dfrac{a_3}{a_2} = \dfrac{a_4}{a_3} = ~ ...$
• The n^th term:
  $a_n = a_m r^{n - m}$
• Sum of the first n terms
  $S_n = \dfrac{a_1(r^n - 1)}{r - 1}$ for $n \gt 1$

  $S_n = \dfrac{a_1(1 - r^n)}{1 - r}$ for $n \lt 1$

Note for Calculator
The STAT mode of calulator y = a · b^x cannot be used for r < 0.