DrDelMath
Graphing a Quadratic Function
with
Positive Leading Coefficient and Discriminant = 0

This is a demonstration of how I anticipate using HTML, CSS, JavaScript, SVG, mathML, and mathJax to provide meaningful online instruction in basic algebra topics as found in Intermediate Algebra and College Algebra.

Problem:Analyze the quadratic function $f$ whose rule is given by

$f(x)=x^2-6x+9={\left(x-3\right)}^2$

Analysis:
The function is a quadratic function so the graph is a parabola which opens up or down.
Because the leading coefficient is positive the parabola opens up.

The fact that the discriminant $b^2-4ac={\left(-6\right)}^2-(4)(1)(9)=0$ is 0, implies the parabola has one x-intercept.
This same information is apparent from the fact that $0=f(x)={\left(x-3\right)}^2$ has only one solution. Which means the parabola has only one x-intercept.
such that it intersects the parabola in one point.
We should also observe that the x-intercept must be the vertex. Therefore the vertex and x-intercepts have the same coordinates (3,0). The x-intercept/vertex should be marked
Because exact values cannot be inferred from a sketch it is important that we label the vertex with exact values for its coordinates.
The graph of the function $f$ is now complete.
All the important information about the function is summarized and displayed in geometic form.

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Interpreting the Graph

The graph shows that the domain is all real numbers R
The graph shows that the range is all real numbers greater than or equal to 0. The range is $[0,+\infty )$

The graph clearly shows the answer to
and because $f(x)=x^2-6x+9$ the answer to Where is $f(x)=0$? is also the solution set for the equation in one variable $x^2-6x+9=0$.

The graph clearly shows the answer to
and because $f(x)=x^2-6x+9$ the answer to Where is $f(x)<0$? is also the solution set for the inequality in one variable $x^2-6x+9<0$.

The graph clearly shows the answer to
and because $f(x)=x^2-6x+9$ the answer to Where is $f(x)>0$? is also the solution set for the inequality in one variable $x^2-6x+9>0$

 

 

A Few Observations About the Interpretation
Definition:The graph of a function $f$ consists of all the points and only the points of the form $\left(a,f\left(a\right)\right)$ where $a$ is a domain element and $f\left(a\right)$ is the unique range element associated with $a$.

In the coordinate plane a point is above the x-axis if and only if its second coordinate is positive (greater than $0$).
In the coordinate plane a point is on the x-axis if and only if its second coordinate is equal to $0$.
In the coordinate plane a point is below the x-axis if and only if its second coordinate is negative (less than $0$).
When we compare second coordinates to $0$ The Law of Trichotomy yields three cases (greater than $0$, equal to $0$, and less than $0$) and those three cases match up nicely with geometric properties (above, on, or below the x-axis) of the Cartesian Coordinate System.

The definition of graph of a function informs us that the second coordinate of every point on the graph is of the form $f\left(a\right)$. That fact together with the observations related to points in the plane permits the following.
A point $\left(a,f\left(a\right)\right)$ on the graph of $f$ is above the x-axis if and only if $f\left(a\right)>0$.
A point $\left(a,f\left(a\right)\right)$ on the graph of $f$ is on the x-axis if and only if $f\left(a\right)=0$.
A point $\left(a,f\left(a\right)\right)$ on the graph of $f$ is below the x-axis if and only if $f\left(a\right)<0$.
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Copyright 2014 by Delano P. Wegener, Ph.D. All Rights Reserved.