This document discusses mathematical induction. It provides two key steps of mathematical induction: 1) the statement P(1) is true, and 2) if P(k) is true, then P(k+1) is also true. It then gives an example of using mathematical induction to prove that the formula 1 + 2 + 3 + ... + n = n(n+1)/2 is true for all positive integers n. Specifically, it shows that P(1) is true, and that if P(k) is true then P(k+1) is also true, thereby proving the formula using mathematical induction.