*f*(*x*) =*a**x*^{2}+*b**x*+*c*,*f*(*x*) =*a*(*x*−*h*)^{2}+*k*.

You can move from the second form to the first by expanding; you can move from the first to the second by completing the square or by using these formulas:

*h*= −*b*/(2*a*);*k*=*f*(*h*) =*c*−*b*^{2}/(4*a*).

If (as we assume) it's not linear,
then the graph of a quadratic function
is a shape called a **parabola**.
The point (*h*, *k*) on the graph
is called the **vertex** of the parabola.
If *a* > 0,
then the vertex gives the *absolute minimum* of the function;
if *a* < 0,
then the vertex gives the *absolute maximum* of the function.
(That is, the absolute minimum or maximum is *k*,
and this absolute extremum occurs at *h*.)
The parabola is symmetric,
with a vertical **line of symmetry**
whose equation is *x* = *h*.
The *initial value* of the function is *f*(0) = *c*,
so the *vertical intercept*, or *y*-intercept, on the graph
is (0, *c*).
The *roots* (or *zeroes*) of the function
(which correspond to
the *horizontal intercepts*, or *x*-intercepts,
on the graph)
are given by the **quadratic formula**:

*r*_{±}=*h*± √(-*k*/*a*) = [−*b*± √(*b*^{2}− 4*a**c*)]/(2*a*).

In general, there are up to 7 useful points on the graph:

- (
*h*,*k*) ―the vertex; - (0,
*c*) ―the vertical intercept; - (2
*h*,*c*); - (
*h*+ 1,*k*+*a*); - (
*h*− 1,*k*+*a*); - (
*r*_{−}, 0) ―one horizontal intercept; - (
*r*_{+}, 0) ―the other horizontal intercept.

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This web page was written between 2010 and 2017 by Toby Bartels, last edited on 2017 August 14. Toby reserves no legal rights to it.

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