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).