Geometric progressions are a list of non-zero numbers where the ratio between each pair of numbers stays the same. Each part of the sequence is made by multiplying the previous part by a constant called the "Common ratio" and adding that to the previous part. The ratio that stays the same in a geometric progression is: r = a2/a1 For the nth term of a GP, the formula is: a = a1rn-1, where an is the first term in the GP and r is the common ratio. The formula for the sum of a GP is: Sn=a1(1-rn)/(1-r), where an is the first term, r is the common ratio, and n is the number of terms.