Chapter 4.1, Problem 106AYU

In Problems 107-114, analyze each polynomial function by following Steps 1 through 8 on page 193.

$f\left(x\right)=-x^5+5x^4+4x^3-20x^2$

To determine

To analyze: The polynomial function.

Answer to Problem 106AYU

$\text{The polynomial function }f\left(x\right)=-x^5+5x^4+4x^3-20x^2\text{ is analyzed}$.

Explanation of Solution

Given:

$f\left(x\right)=-x^5+5x^4+4x^3-20x^2$

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

$f\left(x\right)=-x^5+5x^4+4x^3-20x^2$

Factor

$f\left(x\right)=-x^5+5x^4+4x^3-20x^2=-x^2\left(x+2\right)\left(x-5\right)\left(x-2\right)$.

The polynomial function $f$ is of degree 5. The graph of $f$ behaves like $y=x^5$ 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 $x\text{-intercept }=f\left(x\right)=0$.

$f\left(x\right)=-x^2\left(x+2\right)\left(x-5\right)\left(x-2\right)$

$x=0,0,-2,+5,+2$

The $x\text{-intercept }=x=0,0,-2,+5,+2$.

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 $f=0,0,-2,+5,+2$.

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

The zero $-2$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

The zero $5$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

The zero $2$ has odd multiplicity therefore the graph of $f$ crosses the $X\text{-axis}$.

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

$y_1=-x^5+5x^4+4x^3-20x^2$

Step 5: Approximate the turning point of the graph.

From the graph of $f$, we see $f$ has 4 turning .

Using the graph maximum turning point $=\left(0,0\right)$.

Using the graph minimum turning point $=\left(1.6,-14.2\right)$.

Step 6: Redraw the graph.

The graph passes through $\left(2,0\right)\left(0,0\right)\left(-2,0\right)\left(5,0\right)$.

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 $\left(-\infty ,0\right]$ and $\left[1.6,\infty \right)$.

Based on the graph, $f$ is decreasing $\left[0,1.6\right]\left[-2,-\infty \right)$.