The 10th and the 20th term of an arithmetic sequence are 10 and 40, respectively. Find its 100th term.

A 10 = 10;

a 20 = 40;

a n = a 1 + (n - 1) * r = > 10 = a 1 + 9 * r, and, 40 = a 1 + 19 * r = > 30 = 10 * r = > r = 3 = > a 1 = 10 - 27 = > a 1 = - 17;

a 100 = a 1 + 99 * r = > a 100 = - 17 + 99 * 3 = > a 100 = - 17 + 297 = > a 100 = 280.