General forms of a sequence

 Arithmetic---the same number is added or subtracted from each element to get the next element in the sequence.

Examples--- 3,7,11,15,19

Formula--- a_n = a₁ + (n – 1)d

Geometric---- the same number is multiplied or divided by each element to get the next element in the sequence.

Examples----2, 6, 18, 54

Formula---    a_n = a₁ ^. r^{(n - 1)}