Problem 26 - Contemporary Precalculus

Question

Sketch the graph of the equation and label the vertices. $$r=\frac{12}{3+4 \sin \theta}$$

Step-by-Step Solution

Answer

Question: Sketch the polar graph for the equation $$r = \frac{12}{3 + 4\sin\theta}$$ and find the minimum and maximum points of the graph. Answer: The graph of the given polar equation has a loop structure with two key vertices (minimum and maximum points). The minimum point is (3, π/2) and the maximum point is (-4, 3π/2).

Step 1: Determine the range for θ

Since we are sketching a polar equation, we will consider the range for θ from 0 to 2π (0° to 360°) as the polar equation he graph repeats itself after every 2π.

Step 2: Identify key features

Differentiate r with respect to θ to find the vertices. Vertices are points on the graph where r is either a minimum or maximum. Given the equation, $$r = \frac{12}{3 + 4\sin\theta}$$, first find the derivative r'(θ) using the quotient rule. $$r'(\theta) = \frac{(3 + 4\sin\theta)(0) - (12)(4\cos\theta)}{(3 +4\sin\theta)^2} = \frac{-48\cos\theta}{(3 +4\sin\theta)^2}$$ Now, to find the critical points, set r'(θ) = 0 and solve for θ: $$-\frac{48\cos\theta}{(3 +4\sin\theta)^2} = 0$$ To find the critical points, we have: $$\cos\theta = 0$$ The values of θ that meet this condition when 0 ≤ θ ≤ 2π are: $$\theta = \frac{\pi}{2}, \frac{3\pi}{2}$$ Plug these values into the given equation to get minimum and maximum r_values: $$r(\frac{\pi}{2}) = \frac{12}{3 + 4\sin(\frac{\pi}{2})} = \frac{12}{3+4}=3$$ $$r(\frac{3\pi}{2}) = \frac{12}{3 + 4\sin(\frac{3\pi}{2})} = \frac{12}{3-4}=4$$

Step 3: Sketch the graph and label the vertices

Use the vertices found in step 2 and plot them on polar coordinate plane. Use the range of θ values from 0 to 2π: 1. r = 3 at θ = π/2 2. r = -4 at θ = 3π/2 The graph is illustrated as having a loop structure with the points (3, π/2) being the vertex minimum point (closest to the origin) and (-4, 3π/2) being the vertex maximum point (the farthest from the origin). Now that the graph is sketched, plot the two vertices (3, π/2) and (-4, 3π/2) and label them accordingly.

Key Concepts

Polar Coordinates

Polar coordinates present a unique and powerful way to represent points on a plane. Unlike the traditional Cartesian coordinates that use a grid of vertical and horizontal lines, polar coordinates describe a point's location based on its distance from a central point, known as the pole, and an angle relative to a direction, typically the positive x-axis which is called the polar axis.

In mathematical notation, a point in polar coordinates is written as \( (r, \theta) \), where \( r \) is the radius or the distance from the pole, and \( \theta \) is the angle in radians measured from the polar axis. It's essential to become comfortable with this coordinate system as it's frequently used in fields such as physics, engineering, and mathematics, particularly when dealing with problems that exhibit radial symmetry.

Critical Points in Polar Equations

Critical points in the context of polar equations are the angles where the radius—denoted \( r \)—takes on local maximum, minimum, or it changes direction, leading to various features such as loops or cusps on the graph. To find these critical points, you need to take the derivative of the polar equation with respect to \( \theta \) and set it to zero. This helps in identifying at which angles, \( \theta \), the function has maximums or minimums, which are considered critical because they are points where the behavior of the graph changes.

The critical points give valuable information about the graph's shape beyond mere location. For example, when \( r'(\theta) \)= 0, you might find a loop (like in our exercise) or a cusp, which are significant features of polar graphs. Once these points are found, they must be checked against the original equation to determine the corresponding \( r \) values, which are then plotted to help in accurately sketching the graph.

Sketching Polar Graphs

Sketching polar graphs can be refreshing for students accustomed to the Cartesian system. The process involves plotting points and understanding the behavior of the graph at certain critical points. Beginning with a polar coordinate grid is essential, as it has circles for each value of the radius and lines radiating outwards for the angles. When you graph a polar equation, like \( r = \frac{12}{3 + 4\sin \theta} \), you vary \( \theta \) all the way around the pole from 0 to \( 2\pi \) and plot each corresponding \( r \) value.

As demonstrated in the exercise, you should find critical points where \( r \) is a maximum or minimum, which will help shape the graph. Pay special attention to identifying loops, cusps, and intercepts. It may also be helpful to plot several points between critical points to ensure you're capturing the graph's overall shape. Labeling these critical points on the graph contributes to a better understanding of the polar function's behavior.