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 $f(x)=3{x}^{2}+4x+2$

**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 ${b}^{2}-4ac={4}^{2}-(4)(3)(2)=16-24<0$
is negative, implies the parabola does not have an x-intercept.

such that it does not intersect the parabola.

The vertex is an important point and should be marked

Because exact values cannot be inferred from a sketch it is important that we label the vertex with exact values of its coordinates.

Use the fact that the vertex is $\left(\frac{-b}{2a},f\left(\frac{-b}{2a}\right)\right)=\left(\frac{-2}{3},f\left(\frac{-2}{3}\right)\right)=\left(\frac{-2}{3},\frac{2}{3}\right)$

The graph of the function $f$ is now complete.

All the important information about the function is summarized and displayed in geometic form.

**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
$\frac{2}{3}$. The range is
$\left[\frac{2}{3},\infty \right)$

The graph clearly shows the answer to

and because
$f(x)=3{x}^{2}+4x+2$
the answer to Where is
$f(x)=0$?
is also the solution set for the equation in one variable
$3{x}^{2}+4x+2=0$.

The graph clearly shows the answer to

and because
$f(x)=3{x}^{2}+4x+2$
the answer to Where is
$f(x)<0$? is also the solution set for the inequality in one variable $3{x}^{2}+4x+2<0$.

The graph clearly shows the answer to

and because
$f(x)=3{x}^{2}+4x+2$
the answer to Where is
$f(x)>0$? is also the solution set for the inequality in one variable $3{x}^{2}+4x+2>0$

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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