Behaviors of Functions
Learning Outcomes
- Determine where a function is increasing, decreasing, or constant
- Find local extrema of a function from a graph
- Describe behavior of the toolkit functions
A General Note: Local Minima and Local Maxima
A function [latex]f[/latex] is an increasing function on an open interval if [latex]f\left(b\right)>f\left(a\right)[/latex] for any two input values [latex]a[/latex] and [latex]b[/latex] in the given interval where [latex]b>a[/latex]. A function [latex]f[/latex] is a decreasing function on an open interval if [latex]f\left(b\right)<f\left(a\right)[/latex] for any two input values [latex]a[/latex] and [latex]b[/latex] in the given interval where [latex]b>a[/latex]. A function [latex]f[/latex] has a local maximum at [latex]x=b[/latex] if there exists an interval [latex]\left(a,c\right)[/latex] with [latex]a<b<c[/latex] such that, for any [latex]x[/latex] in the interval [latex]\left(a,c\right)[/latex], [latex]f\left(x\right)\le f\left(b\right)[/latex]. Likewise, [latex]f[/latex] has a local minimum at [latex]x=b[/latex] if there exists an interval [latex]\left(a,c\right)[/latex] with [latex]a<b<c[/latex] such that, for any [latex]x[/latex] in the interval [latex]\left(a,c\right)[/latex], [latex]f\left(x\right)\ge f\left(b\right)[/latex].Example: Finding Increasing and Decreasing Intervals on a Graph
Given the function [latex]p\left(t\right)[/latex] in the graph below, identify the intervals on which the function appears to be increasing.Answer: We see that the function is not constant on any interval. The function is increasing where it slants upward as we move to the right and decreasing where it slants downward as we move to the right. The function appears to be increasing from [latex]t=1[/latex] to [latex]t=3[/latex] and from [latex]t=4[/latex] on. In interval notation, we would say the function appears to be increasing on the interval [latex](1,3)[/latex] and the interval [latex]\left(4,\infty \right)[/latex].
Analysis of the Solution
Notice in this example that we used open intervals (intervals that do not include the endpoints), because the function is neither increasing nor decreasing at [latex]t=1[/latex] , [latex]t=3[/latex] , and [latex]t=4[/latex] . These points are the local extrema (two minima and a maximum). [ohm_question]4084[/ohm_question]Tip for success: increasing/decreasing behavior
The behavior of the function values of the graph of a function is read over the x-axis, from left to right. That is,- a function is said to be increasing if its function values increase as x increases;
- a function is said to be decreasing if its function values decrease as x increases.
Example: Finding Local Extrema from a Graph
Graph the function [latex]f\left(x\right)=\dfrac{2}{x}+\dfrac{x}{3}[/latex]. Then use the graph to estimate the local extrema of the function and to determine the intervals on which the function is increasing.Answer: Using technology, we find that the graph of the function looks like that below. It appears there is a low point, or local minimum, between [latex]x=2[/latex] and [latex]x=3[/latex], and a mirror-image high point, or local maximum, somewhere between [latex]x=-3[/latex] and [latex]x=-2[/latex].
Analysis of the Solution
Most graphing calculators and graphing utilities can estimate the location of maxima and minima. The graph below provides screen images from two different technologies, showing the estimate for the local maximum and minimum. Based on these estimates, the function is increasing on the interval [latex]\left(-\infty ,-{2.449}\right)[/latex] and [latex]\left(2.449\text{,}\infty \right)[/latex]. Notice that, while we expect the extrema to be symmetric, the two different technologies agree only up to four decimals due to the differing approximation algorithms used by each. (The exact locations of the extrema are at [latex]\pm \sqrt{6}[/latex], but determining this requires calculus.)tip for success: reading extrema
Recall that points on the graph of a function are ordered pairs in the form of[latex]\left(\text{input, output}\right) \quad = \quad \left(x, f(x)\right)[/latex].
If a function's graph has a local minimum or maximum at some point [latex]\left(x, f(x)\right)[/latex], we say"the extrema occurs at [latex]x[/latex], and that the minimum or maximum is [latex]f(x)[/latex]."
Try It
Graph the function [latex]f\left(x\right)={x}^{3}-6{x}^{2}-15x+20[/latex] to estimate the local extrema of the function. Use these to determine the intervals on which the function is increasing and decreasing.Answer: The local maximum appears to occur at [latex]\left(-1,28\right)[/latex], and the local minimum occurs at [latex]\left(5,-80\right)[/latex]. The function is increasing on [latex]\left(-\infty ,-1\right)\cup \left(5,\infty \right)[/latex] and decreasing on [latex]\left(-1,5\right)[/latex].
[ohm_question]32572[/ohm_question]Example: Finding Local Maxima and Minima from a Graph
For the function [latex]f[/latex] whose graph is shown below, find all local maxima and minima.Answer: Observe the graph of [latex]f[/latex]. The graph attains a local maximum at [latex]x=1[/latex] because it is the highest point in an open interval around [latex]x=1[/latex]. The local maximum is the [latex]y[/latex] -coordinate at [latex]x=1[/latex], which is [latex]2[/latex]. The graph attains a local minimum at [latex]\text{ }x=-1\text{ }[/latex] because it is the lowest point in an open interval around [latex]x=-1[/latex]. The local minimum is the y-coordinate at [latex]x=-1[/latex], which is [latex]-2[/latex].
[ohm_question]32571[/ohm_question]tip for success: toolkit functions
The toolkit functions continue to appear throughout the course. Have you memorized them yet? Would you be able to sketch a quick graph of each from its equation?Analyzing the Toolkit Functions for Increasing or Decreasing Intervals
We will now return to our toolkit functions and discuss their graphical behavior in the table below.Function | Increasing/Decreasing | Example |
---|---|---|
Constant Function[latex]f\left(x\right)={c}[/latex] | Neither increasing nor decreasing | |
Identity Function[latex]f\left(x\right)={x}[/latex] | Increasing | |
Quadratic Function[latex]f\left(x\right)={x}^{2}[/latex] | Increasing on [latex]\left(0,\infty\right)[/latex]Decreasing on [latex]\left(-\infty,0\right)[/latex] Minimum at [latex]x=0[/latex] | |
Cubic Function[latex]f\left(x\right)={x}^{3}[/latex] | Increasing | |
Reciprocal[latex]f\left(x\right)=\frac{1}{x}[/latex] | Decreasing [latex]\left(-\infty,0\right)\cup\left(0,\infty\right)[/latex] | |
Reciprocal Squared[latex]f\left(x\right)=\frac{1}{{x}^{2}}[/latex] | Increasing on [latex]\left(-\infty,0\right)[/latex]Decreasing on [latex]\left(0,\infty\right)[/latex] | |
Cube Root[latex]f\left(x\right)=\sqrt[3]{x}[/latex] | Increasing | |
Square Root[latex]f\left(x\right)=\sqrt{x}[/latex] | Increasing on [latex]\left(0,\infty\right)[/latex] | |
Absolute Value[latex]f\left(x\right)=|x|[/latex] | Increasing on [latex]\left(0,\infty\right)[/latex]Decreasing on [latex]\left(-\infty,0\right)[/latex] Minimum at [latex]x=0[/latex] |
Use a graph to locate the absolute maximum and absolute minimum of a function
There is a difference between locating the highest and lowest points on a graph in a region around an open interval (locally) and locating the highest and lowest points on the graph for the entire domain. The [latex]y\text{-}[/latex] coordinates (output) at the highest and lowest points are called the absolute maximum and absolute minimum, respectively. To locate absolute maxima and minima from a graph, we need to observe the graph to determine where the graph attains it highest and lowest points on the domain of the function. Not every function has an absolute maximum or minimum value. The toolkit function [latex]f\left(x\right)={x}^{3}[/latex] is one such function.A General Note: Absolute Maxima and Minima
The absolute maximum of [latex]f[/latex] at [latex]x=c[/latex] is [latex]f\left(c\right)[/latex] where [latex]f\left(c\right)\ge f\left(x\right)[/latex] for all [latex]x[/latex] in the domain of [latex]f[/latex]. The absolute minimum of [latex]f[/latex] at [latex]x=d[/latex] is [latex]f\left(d\right)[/latex] where [latex]f\left(d\right)\le f\left(x\right)[/latex] for all [latex]x[/latex] in the domain of [latex]f[/latex].Example: Finding Absolute Maxima and Minima from a Graph
For the function [latex]f[/latex] shown below, find all absolute maxima and minima.Answer: Observe the graph of [latex]f[/latex]. The graph attains an absolute maximum in two locations, [latex]x=-2[/latex] and [latex]x=2[/latex], because at these locations, the graph attains its highest point on the domain of the function. The absolute maximum is the y-coordinate at [latex]x=-2[/latex] and [latex]x=2[/latex], which is [latex]16[/latex]. The graph attains an absolute minimum at [latex]x=3[/latex], because it is the lowest point on the domain of the function’s graph. The absolute minimum is the y-coordinate at [latex]x=3[/latex], which is [latex]-10[/latex].
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- College Algebra. Provided by: OpenStax Authored by: Abramson, Jay et al.. Located at: https://openstax.org/books/college-algebra/pages/1-introduction-to-prerequisites. License: CC BY: Attribution. License terms: Download for free at http://cnx.org/contents/[email protected].
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