The vertex form of a quadratic
is given by
y = a(x – h)2 + k,
where (h, k) is the vertex.
in the vertex form is the same "a"
in y = ax2 + bx + c (that is, both a's
have exactly the same value). The sign on "a"
tells you whether the quadratic opens up or opens down. Think of it this
way: A positive
draws a smiley, and a negative "a"
draws a frowny. (Yes, it's a silly picture to have in your head, but it
makes is very easy to remember how the leading coefficient works.)