Sequences and series
Arithmetic sequences add a common difference, tn = a + (n - 1)d; geometric ones multiply by a common ratio, tn = ar^(n-1).
A worked example
For 4, 10, 16, 22, ... the common difference is 6, so the 8th term is 4 + 7(6) = 46.
The mistake people actually make
The exponent is n - 1, not n, so the 5th term of a geometric sequence uses r^4. Read whether the question wants a single term or the sum of the terms.