Problem 18 - Precalculus

Question

Graph the polar equations. $$r=2-4 \sin \theta$$

Step-by-Step Solution

Answer

It's a Limaçon with an inner loop, plot key points and maintain symmetry about \( \theta = \pi/2 \).

Step 1: Identify the Type of Polar Equation

The equation given is in the form \( r = a + b \cdot \sin \theta \). This is a type of Limaçon. Depending on the values of \( a \) and \( b \), the Limaçon can have different shapes.

Step 2: Determine Limaçon Characteristics

For the equation \( r = 2 - 4 \sin \theta \), \( a = 2 \) and \( b = -4 \). Since \( |b| > |a| \), this indicates a Limaçon with an inner loop.

Step 3: Find Key Points for Plotting

To better understand the graph, calculate the radius \( r \) for key angles such as \( \theta = 0, \pi/2, \pi, \text{ and } 3\pi/2 \). Computing these yields:- \( \theta = 0 \): \( r = 2 \)- \( \theta = \pi/2 \): \( r = -2 \)- \( \theta = \pi \): \( r = 2 \)- \( \theta = 3\pi/2 \): \( r = 6 \)

Step 4: Recognize Symmetry and Patterns

The equation \( r = 2 - 4 \sin \theta \) is symmetric with respect to the \,\( \theta = \pi/2 \) line due to the \( \sin \theta \) term. This symmetry helps in predicting and reflecting points more easily on the graph.

Step 5: Sketch the Limaçon

Plot the points calculated in Step 3 and follow the structure of a Limaçon with an inner loop. Begin at \( \theta = 0 \), progressing to \( \theta = \pi \) as the radius decreases to create and close the loop, and finally reach \( \theta = 2\pi \), maintaining symmetry about the \( \theta = \pi/2 \) line.

Key Concepts

Graphing Limaçons

A limaçon is a fascinating type of polar graph characterized by its distinctive shape, which can vary based on the parameters of its governing equation. The general form of a limaçon equation is \( r = a + b \, \sin \theta \) or \( r = a + b \, \cos \theta \). The relationship between the constants \( a \) and \( b \) primarily determines the shape:

• No inner loop: When \( |a| \geq |b| \), the limaçon does not have an inner loop, appearing more linear or heart-shaped.
• Inner loop: When \( |b| > |a| \), the limaçon modifies into a looped shape, giving rise to an inner loop.
• Cardioid: Specially, when \( |a| = |b| \), the limaçon forms a cardioid, a heart-shaped curve without any inner loops.

In our given equation, \( r = 2 - 4 \, \sin \theta \), the magnitude of \( b \) exceeds \( a \), indicating a limaçon with an inner loop. To sketch the graph, it's crucial to identify points where the radial distance \( r \) changes, considering the full rotation from 0 to \( 2\pi \). Being familiar with these characteristics helps in accurately drawing the distinct curves of a limaçon.

Polar Coordinates

Polar coordinates provide an alternative to the traditional Cartesian coordinates, describing a point's location through the radius and angle rather than \( x \) and \( y \). In the polar system, each point is specified uniquely by \( (r, \theta) \), where:

• \( r \) denotes the distance from the point to the origin (pole).
• \( \theta \) represents the angle from the polar axis, usually directed towards the positive x-axis.

This system excels in graphing curves involving circular or rotational symmetry. For instance, graphing the equation \( r = 2 - 4 \, \sin \theta \) involves plotting points where \( r \) changes as \( \theta \) moves through defined key angles like 0, \( \pi/2 \), \( \pi \), and \( 3\pi/2 \). This concise approach, coupled with recognizing symmetry about specific lines (such as \( \theta = \pi/2 \)), simplifies the visual representation and interpretation of polar plots. Understanding polar coordinates equips students and researchers with a robust tool for tackling a variety of mathematical and scientific problems.

Trigonometric Functions

Trigonometric functions like sine and cosine are indispensable in polar equations because they describe the oscillations and repetitive patterns that define many natural phenomena. In our context, \( \sin \theta \) plays a vital role in shaping the limaçon's curve.The function \( \sin \theta \) exhibits the following essential characteristics:

• It completes one full cycle in the interval \( [0, 2\pi] \).
• It oscillates with values ranging from -1 to 1.
• Key points occur at angles \( \theta = 0, \pi/2, \pi, \text{ and } 3\pi/2 \), where the sine values are 0, 1, 0, and -1, respectively.

For the given polar equation \( r = 2 - 4 \, \sin \theta \), the sin term produces variations in \( r \) that influence the shape, producing an inner loop for values where \( \sin \theta \) becomes negative enough to make \( r \) negative. Recognizing these patterns allows us to predict how changing \( \theta \) shifts \( r \), thereby controlling the graph's symmetry and form. Mastering how trigonometric functions manipulate polar equations is integral in both familiarizing oneself with the behavior of complex curves and effectively applying mathematical concepts.