Quadratic Equation
Pronunciation: /kwɒˈdræ.tɪk ɪˈkweɪ.ʃən/ Explain
Click on the blue points on the sliders and drag them to change the figure.
For what value of 'a' does the plot produce a straight line?

Manipulative 1  Quadratic Equation Created with GeoGebra. 
A quadratic equation is an
equation
of a
polynomial
of degree
two. When graphed, a quadratic equation makes a
parabola
with a horizontal directrix.
The standard form of a quadratic equation is
f(x) = ax^{2} + bx
+ c where a,
b and c
are constant coefficients and a ≠ 0. The
equation in figure 1 placed in standard form is
f(x) = x^{2} +
2x + 0. Manipulative 1 shows the graph
of a quadratic equation using the standard from.
Discovery
 Move the slider for a until
a = 0. What is the result?
Why is the condition a ≠ 0 placed on
the quadratic equation?
 What changes in the graph when a is
changed?
 What changes in the graph when b is
changed?
 What changes in the graph when c is
changed?
Examples of Quadratic Equations 
Quadratic equations  Reason 
y = 3x^{2}  2x  4 
Function is a polynomial of degree 2. 
y = x^{2}  3 
Function is a polynomial of degree 2. 
g = b^{2} 
Function is a polynomial of degree 2. 
y = sin( π / 2 )x^{2} + 4 
Function is a polynomial of degree 2. Since the argument of the sine function
is a constant, the function itself is a constant and can be treated like any other
coefficient. 
r = ( t  1 )( t + 3 ) 
The right side of the equation can be expanded using the distributive property
of multiplication over addition and subtraction to yield
r = t^{2} + 2t  3,
which is a polynomial of degree 2. 
Examples of Equations That Are Not Quadratic 
Nonquadratic equations  Reason 
y = 3x^{3} + 2x^{2} 
x + 3 
Function is a polynomial of degree 3, so it is not quadratic. 
y = x^{2}  sin( x ) + 3 
Since sin( x ) is in the equation, it is not
a polynomial. 
w = 3m  4 
Function is a polynomial of degree 1, so it is not quadratic. 
y = log( 2x^{2}  x + 3) 
Since the equation contains a logarithm with a variable argument, it is not a
polynomial. 
y = x( x  2 )( x + 1 ) 
The right side of the equation can be expanded to y =
x^{3}  x^{2}  2x, which is a
polynomial of degree 3. 
Table 1 
Discriminant of a Quadratic Equation
 Figure 2: Discriminants of quadratic equations. 

The discriminant of a
quadratic equation is used to determine if a quadratic equation has real or complex
roots. The expression for the discriminant is
.
If the discriminant is positive, the quadratic equation has two real roots. If the
discriminant is zero, the quadratic equation has one real root. If the
discriminant is negative, the quadratic equation has two complex roots.

For a quadratic equation in the form ,
the solution can be found using the quadratic formula
.
A quadratic equation can have
2 real roots,
1 real root, or
2
complex roots (see
discriminant).
Example 1: Two real roots.
 
Figure 3: Graph of
x^{2} + 2x 
3. 

Example 2: One real root.
 
Figure 4: Graph of
x^{2} + 4x +
4. 

Example 3: Two complex roots.
 
Figure 5: Graph of
x^{2} + 4 =
0. 

Forms of Quadratic Equations
Click on the blue points on the sliders and drag them to change the figure.
The equation is y=ax^2+b. For what values of a is the graph a straight line? Why
 Manipulative 2  Parabola Created with GeoGebra. 

The standard form a parabolic equation is
y = ax^{2} + bx +
c.
Discovery
 Click on the point for the slider labeled 'a'
and drag it. What changes if 'a' is
negative, zero or positive?
 Click on the point for the slider labeled 'b'
and drag it. What changes as 'b' changes?
 Click on the point for the slider labeled 'c'
and drag it. What changes as 'a' changes?

Click on the blue points on the sliders and drag them to change the figure.
For what value of 'a' does the plot produce a straight line?
 Manipulative 3  Quadratic Equation Created with GeoGebra. 

Xintercept
form of a parabolic equation is y = a(
x  x_{1} )( x  x_{2}
) where x_{1} is one
xintercept of the quadratic equation, x_{2}
is the other xintercept, and a indicates how
steep the sides of the quadratic equation are. If
x_{1} = x_{2},
the quadratic equation intercepts the xaxis only once. Not all quadratic equations
can be described using the xintercept form.
Manipulative 3 shows a quadratic equation with elements of the xintercept form.
Discovery
 Click on the blue points labeled xi_{0}
and xi_{1} and drag them. How do
they change the figure?
 Click on the red point on the slider labeled
a and drag it. How does it change the figure?
 Can you use manipulative 3 to show a quadratic equation that does not intercept the
xaxis? Why or why not?

Click on the blue points on the sliders and drag them to change the figure.
Why does the equation plot as a straight line when a=0?
 Manipulative 4  Vertex Form of a Quadratic Equation Created with GeoGebra. 

The vertex form of a parabolic equation is
y  y_{1} = a(
x  x_{1} )^{2}. The vertex of the
quadratic equation is at the point ( x_{1},
y_{1}). a shows how
steep the sides of the quadratic equation are.
Discovery
 Click on the sliders for x_{1}
and y_{1} and drag them. What
changes about the quadratic equation?
 Click on the slider for a and drag it.
What changes about the quadratic equation?
 If y  y_{1} =
a( x  x_{1} )^{2} is
the equation of the quadratic equation, what is the equation of the line of symmetry?

Graphing a Quadratic Equation
The graph of a quadratic equation can be drawn using
the definition of a parabola: All points in a plane
equidistant
from a line, called the directrix, and a point, called the
focus, which is not on the line. Every quadratic equation
also has a
vertex
and a
line of symmetry.
Discovery
 How is the quadratic equation different if the focus and directrix are close
together as opposed to far apart?
 How does the quadratic equation change if the focus is moved
to the left or to the right?
 Click on the focus (purple point) and drag it until the focus is on the directrix.
What happens to the quadratic equation? When this happens, mathematicians say that
the line is a degenerate quadratic equation.
Parts of a Parabola
Click on the blue point on the slider and drag to change the figure. Click on the vertex and drag to change the figure. Click on the check boxes to show and hide parts of the parabola.
 Manipulative 6  Parts of a Parabola Created with GeoGebra. 

The vertex of a quadratic equation is at the inflection point of the quadratic
equation. The inflection point is the point where the parabolic curve changes direction.
In manipulative 6, the vertex is orange.
The distance from the focus to any point on the quadratic equation is the same
as the distance from that point to the directrix. Show the focus and directrix
by clicking on the checkboxes, then drag the blue point on the parabola to
demonstrate this property.

References
 McAdams, David E.. All Math Words Dictionary, quadratic equation. 2nd Classroom edition 201501084799968. pg 148. Life is a Story Problem LLC. January 8, 2015. Buy the book
More Information
 McAdams, David E.. Parabola. allmathwords.org. Life is a Story Problem LLC. 4/17/2009. https://www.allmathwords.org/en/p/parabola.html.
Cite this article as:
McAdams, David E. Quadratic Equation. 4/29/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. https://www.allmathwords.org/en/q/quadraticequation.html.
Image Credits
Revision History
12/21/2018: Reviewed and corrected IPA pronunication. (
McAdams, David E.)
12/3/2018: Removed broken links, updated license, implemented new markup, updated geogebra app. (
McAdams, David E.)
8/7/2018: Changed vocabulary links to WORDLINK format. (
McAdams, David E.)
1/22/2009: Added figure to section on discriminants. (
McAdams, David E.)
12/10/2008: Added section on discriminant, solutions to a quadratic equation. (
McAdams, David E.)
10/19/2008: Initial version. (
McAdams, David E.)