Chapter 9.4, Problem 15E

To determine

To identify: The amplitude and period of the function then graph the function and describe the function of $g$ as a transformation of the graph of its parent function.

Answer to Problem 15E

The amplitude is 1 and period is $\frac{2\pi }{3}$ and graph is shown in figure (1).

Explanation of Solution

Given:

The given function is $g\left(x\right)=\cos 3x$ .

Calculation:

Compare the function $g\left(x\right)=\cos 3x$ with $y=a\cos b\left(x-h\right)+k$ to find amplitude and period of the function.

  $\begin{array}{c}a=1\\ \text{Period}=\frac{2\pi }{b}\\ =\frac{2\pi }{3}\end{array}$

The horizontal shift is $h=0$ and vertical shifts is $k=0$ .

Draw the midline of the graph, $y=0$ .

Find the intercepts at $x=0,\frac{\pi }{6},\frac{\pi }{3},\frac{\pi }{2},\frac{2\pi }{3}$ to draw the graph.

  $\begin{array}{c}\left(0,\cos 0\right)=\left(0,1\right)\\ \left(\frac{\pi }{6},\cos \frac{\pi }{2}\right)=\left(\frac{\pi }{6},0\right)\\ \left(\frac{\pi }{3},\cos \pi \right)=\left(\frac{\pi }{3},-1\right)\\ \left(\frac{\pi }{2},\cos \frac{3\pi }{2}\right)=\left(\frac{\pi }{2},0\right)\end{array}$

And,

  $\left(\frac{2\pi }{3},\cos 2\pi \right)=\left(\frac{2\pi }{3},1\right)$

Maximum point is $\left(0,1\right)$ and minimum point is $\left(\frac{\pi }{3},-1\right)$ .

Draw the graph of the function $g\left(x\right)=\cos 3x$ as follows.

  

Figure (1)

The graph of $g$ is horizontal shrink by a factor of $\frac{1}{3}$ for the graph of $f$ .

Therefore, the amplitude is 1 and period is $\frac{2\pi }{3}$ and graph is shown in figure (1).