Chapter 6: Problem 20
In Exercises 11-24, identify the conic and sketch its graph. $$ r=\frac{5}{-1+2 \cos \theta} $$
Short Answer
Expert verified
The given equation is of a hyperbola. The specific standard form and sketch will depend on calculation results in Step 2.
Step by step solution
01
Identify the Conic Type
From the given equation \(r=\frac{5}{-1+2 \cos \theta}\), we start by identifying the type of conic. It can be inferred that this equation has the form \(r = \frac{p}{1 ± e cos(\theta - \varphi)}\) which is recognized as the polar equation for a conic section with eccentricity e. Here, if e < 1, the conic is an ellipse; if e = 1, it's a parabola; and if e > 1, it's a hyperbola. Here, the eccentricity e equals 2, indicating that the conic is a hyperbola.
02
Express the Equation in Standard Form
For a hyperbola with a horizontal transverse axis, the standard form in Cartesian representation is \(\frac{(x - h)^2}{a^2} - \frac{(y - k)^2}{b^2} = 1\), where [h, k] are the coordinates of the center; a is the distance from the center to a vertex, b is the distance from the center to a co-vertex, and the vertices are located a units to the left and right of the center. In order to get the equation in a standard form, we also need to transform the equation to Cartesian coordinates. We use the formulas \(x = r cos( \theta)\) and \(y = r sin( \theta)\). So, we substitute \(r = x/cos( \theta)\) into the r given in the exercise. We then collect the terms and adjust the format of the equation. Let's denote a=√(-p/e) and b=√(-p/(e^2 - 1)) and we can find the center coordinates (h, k).
03
Sketch the Graph
After getting the standard form, the center of the hyperbola, the values of a and b, we can sketch the graph. The center (h, k) is marked on the graph. Then, from the center, move a units horizontally (left or right depending on the equation) to plot the vertices, and b units vertically (up or down) to plot the co-vertices. Next, sketch a rectangle using these vertices and co-vertices as guide points. Then draw the asymptotes of the hyperbola, which are the diagonals of this rectangle. Finally, sketch the hyperbola branches approaching the asymptotes.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Hyperbola
The hyperbola is a fascinating and important type of conic section. Conic sections are the curves derived from the intersection of a plane with a double napped cone. A hyperbola is formed when the plane intersects both naps of the cone.
Here are some key characteristics of a hyperbola:
Here are some key characteristics of a hyperbola:
- It consists of two separate curves called branches, which are mirror images of each other.
- The center is the point that's equidistant from both foci and vertices.
- The vertices are the closest points to each other on the two branches.
- The foci are points located inside each branch and serve as the defining feature of hyperbola.
- Asymptotes, which are lines that the branches approach but never touch, guide the hyperbola's shape.
Polar Equations
Polar equations are a way of expressing conics in terms of a radius and angle. This is particularly useful when dealing with curves that possess central symmetry like circles and conic sections.
The general form of a polar equation for a conic is given by:\[ r = \frac{ep}{1 \, ± \, e \, \cos(\theta \pm \varphi)} \].This equation can describe all types of conics:
The general form of a polar equation for a conic is given by:\[ r = \frac{ep}{1 \, ± \, e \, \cos(\theta \pm \varphi)} \].This equation can describe all types of conics:
- If the eccentricity (e) is more than 1, we have a hyperbola.
- If it equals exactly 1, then the conic is a parabola.
- If it's less than 1, the shape is an ellipse.
Eccentricity
Eccentricity is a critical parameter in identifying the type of conic section. It helps in distinguishing between ellipses, parabolas, and hyperbolas. The eccentricity (e) measures how much a conic section deviates from being circular.
Here’s what different values of eccentricity indicate:
Here’s what different values of eccentricity indicate:
- If \( e = 0 \), the conic is a circle, which is a special case of an ellipse.
- If \( 0 < e < 1 \), the conic is an ellipse.
- If \( e = 1 \), we have a parabola.
- If \( e > 1 \), the conic becomes a hyperbola.
Cartesian Coordinates
Cartesian coordinates allow us to describe conics in a more traditional x-y plane, which is very intuitive and widely used. For hyperbolas, converting from polar equations to Cartesian coordinates involves using trigonometric relationships.
In the Cartesian plane, a hyperbola with a horizontal transverse axis has the standard form:\[ \frac{(x - h)^2}{a^2} - \frac{(y - k)^2}{b^2} = 1 \].
This equation highlights several important components:
In the Cartesian plane, a hyperbola with a horizontal transverse axis has the standard form:\[ \frac{(x - h)^2}{a^2} - \frac{(y - k)^2}{b^2} = 1 \].
This equation highlights several important components:
- The center of the hyperbola, located at \((h, k)\).
- \(a\) represents the distance from the center to each vertex horizontally.
- \(b\) denotes the distance from the center to a co-vertex vertically.
- The asymptotes, which can also be derived from this equation.