The coefficients of the quadratic function f(x)=ax²+bx+c are chosen from the numbers 1 through 6 by three independent rolls of a fair die. What is the probability that the parabolic graph of  y=f(x) has its vertex on the x-axis?

Solution:  5/216 . There are  6³=216 possible outcomes for the ordered triple of coefficients (a, b, c). The vertex of the graph will lie on the x-axis if and only if the discriminant b²-4ac=0. Since b must therefore be even, the possibilities can be narrowed down quickly to these five:

(1, 2, 1), (2, 4, 2), (1, 4, 4), (4, 4, 1), and (3, 6, 3).