APEX Calculus: for University of Lethbridge

Start with a point \(O\) in the plane called the pole (we will always identify this point with the origin). From the pole, draw a ray, called the initial ray (we will always draw this ray horizontally, identifying it with the positive \(x\)-axis). A point \(P\) in the plane is determined by the distance \(r\) that \(P\) is from \(O\text{,}\) and the angle \(\theta\) formed between the initial ray and the segment \(\overline{OP}\) (measured counter-clockwise). We record the distance and angle as an ordered pair \((r,\theta)\text{.}\) To avoid confusion with rectangular coordinates, we will denote polar coordinates with the letter \(P\text{,}\) as in \(P(r,\theta)\text{.}\) This is illustrated in Figure 10.4.2

Practice will make this process more clear.

Consider the following two points: \(A = P(1,\pi)\) and \(B = P(-1,0)\text{.}\) To locate \(A\text{,}\) go out 1 unit on the initial ray then rotate \(\pi\) radians; to locate \(B\text{,}\) go out \(-1\) units on the initial ray and don’t rotate. One should see that \(A\) and \(B\) are located at the same point in the plane. We can also consider \(C=P(1,3\pi)\text{,}\) or \(D = P(1,-\pi)\text{;}\) all four of these points share the same location.

This ability to identify a point in the plane with multiple polar coordinates is both a “blessing” and a “curse.” We will see that it is beneficial as we can plot beautiful functions that intersect themselves (much like we saw with parametric functions). The unfortunate part of this is that it can be difficult to determine when this happens. We’ll explore this more later in this section.

It is useful to recognize both the rectangular (or, Cartesian) coordinates of a point in the plane and its polar coordinates. Figure 10.4.5 shows a point \(P\) in the plane with rectangular coordinates \((x,y)\) and polar coordinates \(P(r,\theta)\text{.}\) Using trigonometry, we can make the identities given in the following Key Idea.

Defining a new coordinate system allows us to create a new kind of function, a polar function. Rectangular coordinates lent themselves well to creating functions that related \(x\) and \(y\text{,}\) such as \(y=x^2\text{.}\) Polar coordinates allow us to create functions that relate \(r\) and \(\theta\text{.}\) Normally these functions look like \(r=f(\theta)\text{,}\) although we can create functions of the form \(\theta = f(r)\text{.}\) The following examples introduce us to this concept.

The basic rectangular equations of the form \(x=h\) and \(y=k\) create vertical and horizontal lines, respectively; the basic polar equations \(r= h\) and \(\theta =\alpha\) create circles and lines through the pole, respectively. With this as a foundation, we can create more complicated polar functions of the form \(r=f(\theta)\text{.}\) The input is an angle; the output is a length, how far in the direction of the angle to go out.

We sketch these functions much like we sketch rectangular and parametric functions: we plot lots of points and “connect the dots” with curves. We demonstrate this in the following example.

Technology Note: Plotting functions in this way can be tedious, just as it was with rectangular functions. To obtain very accurate graphs, technology is a great aid. Most graphing calculators can plot polar functions; in the menu, set the plotting mode to something like polar or POL, depending on one’s calculator. As with plotting parametric functions, the viewing “window” no longer determines the \(x\)-values that are plotted, so additional information needs to be provided. Often with the “window” settings are the settings for the beginning and ending \(\theta\) values (often called \(\theta_{\text{ min } }\) and \(\theta_{\text{ max } }\)) as well as the \(\theta_{\text{ step } }\) — that is, how far apart the \(\theta\) values are spaced. The smaller the \(\theta_{\text{ step } }\) value, the more accurate the graph (which also increases plotting time). Using technology, we graphed the polar function \(r=1+\cos(\theta)\) from Example 10.4.11 in Figure 10.4.13.

It is sometimes desirable to refer to a graph via a polar equation, and other times by a rectangular equation. Therefore it is necessary to be able to convert between polar and rectangular functions, which we practice in the following example. We will make frequent use of the identities found in Key Idea 10.4.6.

Some curves have very simple polar equations but rather complicated rectangular ones. For instance, the equation \(r=1+\cos(\theta)\) describes a cardioid (a shape important the sensitivity of microphones, among other things; one is graphed in the gallery in the Limaçon section). It’s rectangular form is not nearly as simple; it is the implicit equation \(x^4+y^4+2x^2y^2-2xy^2-2x^3-y^2=0\text{.}\) The conversion is not “hard,” but takes several steps, and is left as a problem in the Exercise section.

There are a number of basic and “classic” polar curves, famous for their beauty and/or applicability to the sciences. This section ends with a small gallery of some of these graphs. We encourage the reader to understand how these graphs are formed, and to investigate with technology other types of polar functions.

Symmetric about \(x\)-axis: \(r=a\pm b\cos(\theta)\)

Symmetric about \(y\)-axis: \(r=a\pm b\sin(\theta)\text{;}\) \(a,b \gt 0\)

Symmetric about \(x\)-axis: \(r=a \cos(n\theta)\)

Symmetric about \(y\)-axis: \(r=a\sin(n\theta)\)

Curve contains \(2n\) petals when \(n\) is even and \(n\) petals when \(n\) is odd.

Earlier we discussed how each point in the plane does not have a unique representation in polar form. This can be a “good” thing, as it allows for the beautiful and interesting curves seen in the preceding gallery. However, it can also be a “bad” thing, as it can be difficult to determine where two curves intersect.

If all one is concerned with is the \((x,y)\) coordinates at which the graphs intersect, much of the above work is extraneous. We know they intersect at \((0,0)\text{;}\) we might not care at what \(\theta\) value. Likewise, using \(\theta =2\pi/3\) and \(\theta=4\pi/3\) can give us the needed rectangular coordinates. However, in the next section we apply calculus concepts to polar functions. When computing the area of a region bounded by polar curves, understanding the nuances of the points of intersection becomes important.