Problem 39 - Precalculus : Building Concepts and Connections

Question

In Exercises \(31-46,\) sketch the graphs of the polar equations. $$r^{2}=4 \cos (2 \theta)$$

Step-by-Step Solution

Answer

The graph of the polar equation \(r^{2}=4 \cos (2 \theta)\) forms a circle with loops at \(r = +2\) and \(r = -2\), representing a limaçon.

Step 1: Understand the polar equation

In the equation \(r^{2}=4 \cos (2 \theta)\), \(r\) is the radial distance from the origin and \(\theta\) is the direction angle. The equation represents a limaçon, which is a looped or spiral shape.

Step 2: Plot values

To visualize the graph, create a table of values for \(\theta\), evaluating \(r\) for each. For \(\theta=\) 0, \(\pi/4\), \(\pi/2\), \(3\pi/4\), \(\pi\), \(5\pi/4\), \(3\pi/2\), \(7\pi/4\), and \(2\pi\), determine \(r=\sqrt{4\cos(2\theta)}\).

Step 3: Draw the graph

Using the table of values for \(\theta\) and \(r\), sketch the graph in polar coordinates. Given that the value of \(r\) oscillates between \(-2\) and \(2\), the graph forms a circle with loops at \(r = +2\) and \(r = -2\).

Key Concepts

Limaçon

The limaçon is an intriguing figure in mathematics, easily recognized by its distinctive 'snail-like' shape, which includes loops and dimples. It's part of a family of curves known as conic sections. The limaçon is described by polar equations of the form
\( r = a + b \cos(\theta) \) or \( r = a + b \sin(\theta) \), where \( a \) and \( b \) are constants that determine its size and number of loops. When graphing the limaçon, you'll notice that it may present a simple loop, an inner loop or even devoid of a loop, transforming into a cardioid or a dimpled circle, depending on the ratios between \( a \) and \( b \). This variety makes the limaçon a great example to explore the rich geometry of polar equations.

Polar Coordinates

Polar coordinates offer a different perspective for graphing and analyzing shapes. Unlike the familiar Cartesian coordinate system which locates points based on horizontal (\( x \) axis) and vertical distance (\( y \) axis), polar coordinates use
\( r \) (the radial distance from the origin) and \( \theta \) (the direction angle or azimuth) to specify a point's location. This system is particularly useful when dealing with circular and spiral patterns, as it aligns closely with their natural geometry. To plot a point in polar coordinates, you start at the origin, move outward a distance of \( r \) and rotate counterclockwise by \( \theta \) degrees or radians.

Radial Distance

In the context of polar graphs, the radial distance, denoted as \( r \) is a measure of how far a point is from the pole or origin. The pole corresponds to the center of the polar coordinate system. The value of
\( r \) can be positive or negative, indicating the direction along the line defined by the angle \( \theta \). Positive \( r \) values mean you plot the point in the direction \( \theta \) points towards, while negative \( r \) values mean you plot the point in the opposite direction. Radial distance directly influences the appearance of a polar graph, as in the limaçon equation \( r^2 = 4\cos(2\theta) \) where \( r \) oscillates and helps form the looped shape characteristic of the limaçon.

Direction Angle

The direction angle, represented by \( \theta \) in polar coordinates, specifies the angle at which the radial line extends from the origin to a point. This angle is measured from the positive x-axis (also known as the polar axis) in the counterclockwise direction. The direction angle is crucial for plotting points in the polar plane as it aligns the radial distance correctly. In the exercise \( r^2 = 4\cos(2\theta) \), varying \( \theta \) along specific values like 0, \(\frac{\pi}{4}\), \(\frac{\pi}{2}\), shows how the graph of the limaçon takes shape around the origin, as each \( \theta \) corresponds to a different \( r \) and thus a different point on the graph.