10.4 Polar coordinates: graphs  (Page 4/16)

First, testing the equation for symmetry, we find that the graph of this equation will be symmetric about the polar axis. Next, we find the zeros and maximums. Setting $r=0,$ we have $\theta =\pi +2k\pi .$ The zero of the equation is located at $\left(0,\pi \right).$ The graph passes through this point.

The maximum value of $r=2+2\cos \theta$ occurs when $\cos \theta$ is a maximum, which is when $\cos \theta =1$ or when $\theta =0.$ Substitute $\theta =0$ into the equation, and solve for $r.$

The point $(4,0)$ is the maximum value on the graph.

We found that the polar equation is symmetric with respect to the polar axis, but as it extends to all four quadrants, we need to plot values over the interval $[0,\pi ].$ The upper portion of the graph is then reflected over the polar axis. Next, we make a table of values, as in [link] , and then we plot the points and draw the graph. See [link] .

First, testing the equation for symmetry, we find that it fails all three symmetry tests, meaning that the graph may or may not exhibit symmetry, so we cannot use the symmetry to help us graph it. However, this equation has a graph that clearly displays symmetry with respect to the line $\theta =\frac{\pi }{2},$ yet it fails all the three symmetry tests. A graphing calculator will immediately illustrate the graph’s reflective quality.

Next, we find the zeros and maximum, and plot the reflecting points to verify any symmetry. Setting $r=0$ results in $\theta$ being undefined. What does this mean? How could $\theta$ be undefined? The angle $\theta$ is undefined for any value of $\sin \theta >1.$ Therefore, $\theta$ is undefined because there is no value of $\theta$ for which $\sin \theta >1.$ Consequently, the graph does not pass through the pole. Perhaps the graph does cross the polar axis, but not at the pole. We can investigate other intercepts by calculating $r$ when $\theta =0.$

So, there is at least one polar axis intercept at $(4,0).$

Next, as the maximum value of the sine function is 1 when $\theta =\frac{\pi }{2},$ we will substitute $\theta =\frac{\pi }{2}$ into the equation and solve for $r.$ Thus, $r=1.$

Make a table of the coordinates similar to [link] .

The graph is shown in [link] .