Consider the quadratic function $q(x) = Ax^2 + Bx +C$. ($A \ne
0$)
This form for a quadratic function is usually described as the
"standard
polynomial" form of $q$.
The number $A$ is referred to as the "leading coefficient" and the
value of $q(0) = C$ gives the second coordinate of the Y-intercept,
$(0,C)$ for the graph of $q$.
The Vertex Form: There is one other
very useful form for the quadratic function $q$ that uses $A$, the
leading coefficient of $q$, and two number $h$ and $k$ to evaluate
$q(x)$.
In this form, $q$ is evaluated by $q(x) = A(x-h)^2 +k $.
This form is called the "vertex" form of $q$. As we will see in
QF.MA, the value $q(h) = k$ is an extreme value for $q$.
Connection between the Standard and
Vertex Forms.
Expanding the vertex form expression gives $q(x) =A(x^2 - 2hx +h^2)
+ k = Ax^2 - 2Ahx + (Ah^2 +k)$.
From this expression we can see the connection between the
coefficients in the standard form and the numbers $h$ and $k$ of the
vertex form.
In the standard form the coefficient of $x$ is $B$, but from the
vertex form it is $-2Ah$.
We can set these equal: $B = -2Ah$ and since $A \ne 0$, we have $h =
-\frac B {2A}$.
Since in the vertex form it is clear that $q(h) = k$, we have $ k =
A h^2 + Bh + C$ while $C = Ah^2 +k$.
Thus - If we know $A,B$, and $C$ for the standard form, we can
determine $h$ and $k$ of the vertex form, and conversely, if we know
$A, h$ and $k$ of the vertex form we can find $B$ and $C$ for the
standard form. We summarize this connection in the following
The two forms, as well as how to find the
values of $A,B$ and $C$ from three values for $q$, are covered
thoroughly in most algebra texts.
In later subsections we explore how mapping diagrams for quadratic
functions help us visualize these forms and understand further the
meaning of the shape of the graph of $q$.
To be added ?... A dynamic example
to visualize finding $A$ and the vertex form of a quadratic function
with a mapping diagram of $f$ and the parabola in the graph of
$f$.