Chapter 10: Problem 18
Find the slopes of the curves at the given points. Sketch the curves along with their tangents at these points. Cardioid \(\quad r=-1+\sin \theta ; \quad \theta=0, \pi\)
Short Answer
Expert verified
Slopes: \(-1\) at \( (-1,0) \) and \(1\) at \( (1,0) \).
Step by step solution
01
Convert Polar Equation to Cartesian Equation
The polar equation of the cardioid is given as \( r = -1 + \sin \theta \). First, we need to convert this into a Cartesian equation. Using the polar-to-Cartesian conversions, \( x = r \cos \theta \) and \( y = r \sin \theta \), substitute \( r = -1 + \sin \theta \) into these equations. This gives: \( x = (-1 + \sin \theta) \cos \theta \) and \( y = (-1 + \sin \theta) \sin \theta \).
02
Determine the Cartesian Coordinates
Now, evaluate the Cartesian coordinates for the given values of \( \theta \). For \( \theta = 0 \):- \( \sin 0 = 0 \) and \( \cos 0 = 1 \), so \( r = -1 \).- Thus, \( x = -1 \cdot 1 = -1 \) and \( y = -1 \cdot 0 = 0 \). So, the point is \((-1, 0)\).For \( \theta = \pi \):- \( \sin \pi = 0 \) and \( \cos \pi = -1 \), so \( r = -1 \).- Thus, \( x = -1 \cdot (-1) = 1 \) and \( y = -1 \cdot 0 = 0 \).So, the point is \((1, 0)\).
03
Calculate the Derivative and Determine Slope
The slope of the tangent line to a polar curve is given by the formula \( \frac{dr}{d\theta} = \frac{x' \sin\theta + r\cos\theta} {x'\cos\theta - r\sin\theta} \) where \( x' = \frac{dx}{d\theta} \) and \( y' = \frac{dy}{d\theta} \). First, find \( x' = \frac{d}{d\theta}((-1 + \sin\theta )\cos\theta) \) and \( y' = \frac{d}{d\theta}((-1 + \sin\theta) \sin\theta) \).**For \( \theta = 0 \):**- Differentiate to get: \( x' = \cos^2\theta - (1-\sin\theta)\sin\theta = 1 \) and \( y' = \cos\theta\sin\theta = 0 \).- Therefore, the slope is \( m = \frac{0(-1) + (-1)1}{1(1) - (-1)0} = -1 \).**For \( \theta = \pi \):**- Differentiate to find: \( x' = -1 \) and \( y' = 0 \).- Slope here is \( m = \frac{0(1) + (-1)(-1)}{-1(-1) - (-1)0} = 1 \).
04
Sketch the Curve and Tangents
Sketch the cardioid with polar equation \( r = -1 + \sin\theta \). At \( \theta = 0 \), the point is \((-1, 0)\) with a slope of \(-1\). Draw the tangent line at this point with this slope.At \( \theta = \pi \), the point is \((1, 0)\) with a slope of \(1\). Likewise, draw the tangent line at this point. This sketch helps visualize how the cardioid and its tangent lines interact at these points.
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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 describe points in the plane using angles and distances from the origin, instead of the traditional Cartesian coordinates (x, y). Here, each point is represented by
- radius (\(r\)): The distance from the origin
- angle (\(\theta\)): The angle measured counterclockwise from the positive x-axis
- For the x-coordinate: \(x = r \, \cos\, \theta\)
- For the y-coordinate: \(y = r \, \sin\, \theta\)
Cardioid
A cardioid is a particular type of polar curve that resembles a heart shape. Its name stems from the Greek word "kardia," which means "heart." The polar equation describing a cardioid typically takes the form \(r = a + b\sin\, \theta\) or \(r = a + b\cos\, \theta\). In our specific scenario, the cardioid is represented by \(r = -1 + \sin\, \theta\).
- Symmetry: Cardioids often exhibit symmetry about the polar axis, making them a fascinating subject of study in calculus.
- Graph features: The cardioid touches the origin and loops around, demonstrating points of intersection and smooth curves.
Tangent Line
A tangent line to a curve provides the best linear approximation of the curve at a given point, representing the curve's slope or steepness at that point. For polar curves, like our cardioid, the tangent's slope can be calculated using derivative formulas adapted to polar coordinates.
- The slope at a point is determined by the change in the radial distance \(r\) with respect to the angle \(\theta\).
- This slope is computed as \(\frac{dr}{d\theta} = \frac{x' \sin\, \theta + r \cos\, \theta} {x' \cos\, \theta - r \sin\, \theta}\), where \(x'\) and \(y'\) are the derivatives of the Cartesian forms of \(r\cos\, \theta\) and \(r\sin\, \theta\), respectively.
Differentiation
Differentiation is a fundamental tool in calculus for finding the rate at which a quantity changes. It is crucial for understanding how curves behave, especially for finding slopes of tangent lines on curves defined in polar coordinates.
- The process involves computing the derivative of the given function relative to its variable, for polar curves, this is typically with respect to \(\theta\).
- The derivatives of \(x\) and \(y\) in terms of \(\theta\) provide information about the slope of the curve at any point.