Chapter 4.1, Problem 101AYU

To determine

To analyze: The polynomial function.

Answer to Problem 101AYU

$\text{The polynomial function }f\left(x\right)=x^3+x^2-12x\text{ is analyzed}$.

Explanation of Solution

Given:

$f\left(x\right)=x^3+x^2-12x$

Step 1: Determine the end behavior of the graph of the function.

$f\left(x\right)=x^3+x^2-12x$

Factor

$f\left(x\right)=x^3+x^2-12x=x\left(x^2+x-12\right)$

The polynomial function $f$ is of degree 3. The graph of $f$ behaves like $y=x^3$ for large values of $\left|x\right|$.

Step 2: Find $x$ and $y\text{-intercepts}$ of the graph of the function.

The $y\text{-intercept }=f\left(0\right)=0$.

The $x\text{-intercept }=f\left(x\right)=0$.

$f\left(x\right)=x^3+x^2-12x=x\left(x^2+x-12\right)$

$f\left(x\right)=x\left(x^2+4x-3x-12\right)=x\left(x+4\right)\left(x-3\right)$

$x=0\text{ or} x=-4,+3$

The $x\text{-intercept }=x=0,-4,3$.

Step 3: Determine the zeros of the function and their multiplicity. Using this information determine whether the graph crosses or touches the $X\text{-axis}$ at each $x\text{-intercept}$.

The zeros of $f=0,-4,3$.

The zero $-4$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

The zero $0$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

The zero $3$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

Step 4: Using the graphing utility to graph the function.

$y_1=x^3+x^2-12x$

Step 5: Approximate the turning point of the graph.

From the graph of $f$, we see $f$ has 2 turning points.

Using the graph maximum turning point $=\left(-3,22\right)$.

Using the graph minimum turning point $=\left(2,-14\right)$.

Step 6: Redraw the graph.

The graph passes through $\left(0,0\right)\left(-4,0\right)$ and $\left(3,0\right)$.

Step 7: Find the domain and range of the function.

The domain and range is the set of all real numbers.

Step 8: Use the graph to determine where the function is increasing or where it is decreasing.

Based on the graph, $f$ is increasing $\left(-\infty ,-3\right]$ and $\left[1.5,\infty \right)$.

Based on the graph, $f$ is decreasing $\left[-3,1.5\right]$.