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Sia

Question
Math
Posted 2 months ago

The curve above is drawn in the xyx y-plane and is described by the equation in polar coordinates r=θ+sin(2θ)r=\theta+\sin (2 \theta) for 0θπ0 \leq \theta \leq \pi, where rr is measured in meters and θ\theta is measured in radians. The derivative of rr with respect to θ\theta is given by drdθ=1+2cos(2θ)\frac{d r}{d \theta}=1+2 \cos (2 \theta).
(a) Find the area bounded by the curve and the xx-axis.
(b) Find the angle θ\theta that corresponds to the point on the curve with xx-coordinate -2 .
(c) For π3<θ<2π3,drdθ\frac{\pi}{3}<\theta<\frac{2 \pi}{3}, \frac{d r}{d \theta} is negative. What does this fact say about rr ? What does this fact say about the curve?
(d) Find the value of θ\theta in the interval 0θπ20 \leq \theta \leq \frac{\pi}{2} that corresponds to the point on the curve in the first quadrant with greatest distance from the origin. Justify your answer.
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Answer from Sia
Posted 2 months ago
Solution by Steps
step 1
The area bounded by the curve and the xx-axis in polar coordinates is given by the integral: 120πr2dθ\frac{1}{2} \int_{0}^{\pi} r^2 \, d\theta
step 2
Substitute r=θ+sin(2θ)r = \theta + \sin(2\theta) into the integral: 120π(θ+sin(2θ))2dθ\frac{1}{2} \int_{0}^{\pi} (\theta + \sin(2\theta))^2 \, d\theta
step 3
Expand the integrand: 120π(θ2+2θsin(2θ)+sin2(2θ))dθ\frac{1}{2} \int_{0}^{\pi} (\theta^2 + 2\theta \sin(2\theta) + \sin^2(2\theta)) \, d\theta
step 4
Integrate each term separately: 12[0πθ2dθ+20πθsin(2θ)dθ+0πsin2(2θ)dθ]\frac{1}{2} \left[ \int_{0}^{\pi} \theta^2 \, d\theta + 2 \int_{0}^{\pi} \theta \sin(2\theta) \, d\theta + \int_{0}^{\pi} \sin^2(2\theta) \, d\theta \right]
step 5
Evaluate the integrals: 12[π33+0+π2]=12(π33+π2)=π36+π4\frac{1}{2} \left[ \frac{\pi^3}{3} + 0 + \frac{\pi}{2} \right] = \frac{1}{2} \left( \frac{\pi^3}{3} + \frac{\pi}{2} \right) = \frac{\pi^3}{6} + \frac{\pi}{4}


 (b) Find the angle θ\theta that corresponds to the point on the curve with xx-coordinate -2.
step 1
The xx-coordinate in polar coordinates is given by x=rcos(θ)x = r \cos(\theta)
step 2
Substitute r=θ+sin(2θ)r = \theta + \sin(2\theta) and x=2x = -2: 2=(θ+sin(2θ))cos(θ)-2 = (\theta + \sin(2\theta)) \cos(\theta)
step 3
Solve the equation 2=(θ+sin(2θ))cos(θ)-2 = (\theta + \sin(2\theta)) \cos(\theta) for θ\theta


 (c) For π3<θ<2π3\frac{\pi}{3} < \theta < \frac{2\pi}{3}, drdθ\frac{d r}{d \theta} is negative. What does this fact say about rr? What does this fact say about the curve?
step 1
Given drdθ=1+2cos(2θ)\frac{d r}{d \theta} = 1 + 2 \cos(2\theta)
step 2
For π3<θ<2π3\frac{\pi}{3} < \theta < \frac{2\pi}{3}, cos(2θ)\cos(2\theta) is negative, making 1+2cos(2θ)1 + 2 \cos(2\theta) negative
step 3
Since drdθ\frac{d r}{d \theta} is negative, rr is decreasing in this interval
step 4
The curve is moving closer to the origin as θ\theta increases from π3\frac{\pi}{3} to 2π3\frac{2\pi}{3}


 (d) Find the value of θ\theta in the interval 0θπ20 \leq \theta \leq \frac{\pi}{2} that corresponds to the point on the curve in the first quadrant with greatest distance from the origin. Justify your answer.
step 1
The distance from the origin in polar coordinates is given by r=θ+sin(2θ)r = \theta + \sin(2\theta)
step 2
To find the maximum distance, take the derivative and set it to zero: drdθ=1+2cos(2θ)=0\frac{d r}{d \theta} = 1 + 2 \cos(2\theta) = 0
step 3
Solve for θ\theta: 1+2cos(2θ)=0cos(2θ)=122θ=2π3θ=π31 + 2 \cos(2\theta) = 0 \Rightarrow \cos(2\theta) = -\frac{1}{2} \Rightarrow 2\theta = \frac{2\pi}{3} \Rightarrow \theta = \frac{\pi}{3}
step 4
Verify that θ=π3\theta = \frac{\pi}{3} is within the interval 0θπ20 \leq \theta \leq \frac{\pi}{2} and corresponds to a maximum by checking the second derivative or evaluating rr at endpoints
Answer
(a) π36+π4\frac{\pi^3}{6} + \frac{\pi}{4}
(b) Solve 2=(θ+sin(2θ))cos(θ)-2 = (\theta + \sin(2\theta)) \cos(\theta) for θ\theta
(c) rr is decreasing; the curve moves closer to the origin
(d) θ=π3\theta = \frac{\pi}{3}
Key Concept
Polar coordinates and integrals
Explanation
The problem involves finding areas, solving equations, and analyzing behavior in polar coordinates using calculus techniques.

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