<p>Do Arithmetic and Geometric Sequence questions appear on the PSAT, and if so would it be helpful to know the basic formulas for Arithmetic and Geometric Sequences. ex. (n = a1 + (n - 1)(d))?</p>
<p>I think they occasionally appear, but some of the formulas aren’t worth memorizing because they’re either intuitive or easy to mess up. On the LHS it should be a_n, not n.</p>
<p>For example, if it asks you to find the 10th term of the sequence 1,5,9,13,…, realize that you have to add nine “common differences” to get from a<em>1 (1) to a</em>10. Thus the tenth term is 1 + 9(4) = 37.</p>
<p>Same with geometric sequences/series. Note that the sum 1 + r + r^2 + … + r^n is equal to (r^(n+1) - 1)(r - 1). If the sequence is infinite, it converges to 1/(1 - r), provided that 0 < |r| < 1. I don’t know how often they occur on the PSAT, but they’re worth knowing anyway.</p>