Reference for a cubic with a double root?

From: George Baloglou (baloglou_at_panix.com)
Date: 08/10/04 

Date: 9 Aug 2004 21:15:43 -0400

[reply address is baloglouAToswego.edu]

Following the standard derivation of the formula for the solutions of a
cubic, it is not hard to see that ax^3 + bx^2 + cx + d = 0 has a double
root if and only if (2b^3 - 9a*b*c + 27a^2*d)^2 = 4(b^2 - 3a*c)^3. Does
anyone know of a book where this fact is explicitly mentioned and proven?

                                                Thanks in advance,

                                                baloglouAToswego.edu