Chapter 12: Problem 64
Write the polar equation \(3 r=\sin \theta\) as an equation in rectangular coordinates. Identify the equation and graph it.
Short Answer
Expert verified
\[ 3x^2 + 3y^2 - y = 0 \] is a circle.
Step by step solution
01
Identify Polar Equation
Given polar equation: \[ 3r = \sin \theta \]
02
Express Variables
Express the variables in the polar equation in terms of rectangular coordinates. Recall that: \[ x = r \cos \theta \] \[ y = r \sin \theta \] and \[ r = \sqrt{x^2 + y^2} \]
03
Express \sin \theta
Since \sin \theta = \frac{y}{r}, rewrite the given equation \[ 3r = \sin \theta \] as \[ 3r = \frac{y}{r} \]
04
Isolate r
Isolate r by multiplying both sides by r: \[ 3r^2 = y \]
05
Substitute for r
Substitute \sqrt{x^2 + y^2} for r: \[ 3(x^2 + y^2) = y \]
06
Distribute and Rearrange
Distribute 3 and rearrange the equation to standard form: \[ 3x^2 + 3y^2 - y = 0 \]
07
Identify the Equation
Identify the resulting equation. This equation represents a conic section, specifically a circle.
08
Graph the Equation
Plot the graph of the equation \[ 3x^2 + 3y^2 - y = 0 \] This is a circle centered at the origin, but shifted slightly upwards.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Polar Coordinates
In polar coordinates, each point in a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (origin) is often denoted as \(O\), and the reference direction is usually the positive x-axis. The coordinates are given as \( (r, \theta) \), where:
- \(r\): the radial distance from the origin to the point
- \(\theta\): the angle measured counterclockwise from the positive x-axis to the line segment connecting the origin to the point
Rectangular Coordinates
Rectangular coordinates, also known as Cartesian coordinates, use two perpendicular axes: the x-axis and the y-axis. Any point in the plane is represented as \( (x, y) \), where:
- \(x\): the horizontal distance from the origin
- \(y\): the vertical distance from the origin
- \(x = r \cos \theta\)
- \(y = r \sin \theta\)
- \(r = \sqrt{x^2 + y^2}\)
Trigonometric Identities
Trigonometric identities are equations involving trigonometric functions that are true for all values of the angle involved. They are crucial in converting between coordinate systems. In particular:
- \(\sin \theta = \frac{y}{r}\)
Conic Sections
Conic sections are curves obtained by slicing a cone with a plane. They include circles, ellipses, parabolas, and hyperbolas. The equation \(3(x^2 + y^2) = y\) is a form of a conic section. Specifically, simplifying and analyzing it reveals that it represents a circle:
- The general form, \(Ax^2 + By^2 + Cy + D = 0\), identifies different conics.
- In our case, \(3x^2 + 3y^2 - y = 0\) is a circle centered at the origin but slightly shifted due to the y-term.
- Drawing its graph, one can see the symmetric circular shape.