Chapter 8: Problem 16
For the following exercises, identify the conic with a focus at the origin, and then give the directrix and eccentricity. $$ r(4-5 \sin \theta)=1 $$
Short Answer
Expert verified
The conic is a hyperbola with eccentricity 5 and directrix \( d = \frac{1}{5} \).
Step by step solution
01
Recognize the Conic Form
The given polar equation is \( r(4-5 \sin \theta) = 1 \). To identify the type of conic, we can rewrite it in the standard polar form for conics: \( r = \frac{ed}{1 + e\sin\theta} \). Here, the equation resembles \( r = \frac{1}{4 - 5\sin\theta} \), which fits the general form \( r = \frac{ed}{1 - e\sin\theta} \). Thus, \( e \) is 5, and the sign indicates the focus is at the pole.
02
Determine the Eccentricity
In the equation \( r = \frac{1}{4 - 5\sin\theta} \), the eccentricity \( e \) is 5, which is the coefficient of \( \sin \theta \) after rewriting the expression. Since \( e > 1 \), this conic is a hyperbola.
03
Calculate the Directrix
For a conic section in polar coordinates with a focus at the origin, the relationship between the eccentricity \( e \) and the directrix \( d \) is given by \( d = \frac{k}{e} \), where \( k \) is the constant term in the denominator. Here, \( ed = 1 \) implies \( d = \frac{1}{5} \).
04
Identify the Conic and its Parameters
Given the eccentricity \( e = 5 > 1 \), the conic is a hyperbola. The focus is at the origin as stated. The directrix calculated is \( d = \frac{1}{5} \), indicating the line that relates to the vertex of this hyperbola.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Polar Coordinates
Polar coordinates offer a unique way to express locations in a 2-dimensional plane. Unlike Cartesian coordinates which use grid-like x and y axes, polar coordinates rely on a point's distance from a fixed point (called the pole, typically the origin) and an angle from a fixed direction. The position of a point is described using two values:
- \( r \): the radial distance from the pole
- \( \theta \): the angle measured in radians or degrees from the positive x-axis
Eccentricity
Eccentricity is a crucial concept in understanding conic sections, particularly when distinguishing between their different forms. It is symbolized by \( e \) and describes how much a conic section deviates from being circular.The value of eccentricity determines the shape:
- If \( e = 0 \), the conic is a circle.
- If \( 0 < e < 1 \), the conic is an ellipse.
- If \( e = 1 \), the conic is a parabola.
- If \( e > 1 \), the conic is a hyperbola.
Hyperbola
A hyperbola is one of the four types of conic sections. It is formed by intersecting a double cone with a plane at an angle steeper than the angle of the cone, resulting in two separate curves called branches.In polar coordinates, a hyperbola is expressed when the eccentricity \( e > 1 \). This indicates how stretched the hyperbola is compared to a circle. The branches mirror each other, and they diverge, extending out to infinity.The equation from our exercise fits the form: \[ r = \frac{ed}{1 - e\sin\theta} \] Identifying this polar conic as a hyperbola tells us about its open-ended structure and directional properties based on its parameters.
Directrix
The directrix is an essential feature for defining conic sections, especially in polar geometry. It is a fixed line used in conjunction with the focus to generate and analyze different conics by defining their respective shapes and sizes. The relationship between the eccentricity \( e \), the distance to the directrix \( d \), and other parameters is captured in the equation of the conic. Here, the directrix provides a line such that the distance from any point on the conic to the focus is a constant multiple (given by the eccentricity) of the perpendicular distance from the point to the directrix.In our example, since we have \( e = 5 \) and \( ed = 1 \), the directrix is calculated as \( d = \frac{1}{5} \). This line plays a crucial role in maintaining the fixed proportion that defines the hyperbola's shape.