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Question
Math
Posted 9 months ago
Consider the polar curve r=4sin(3θ)r=4 \sin (3 \theta).
What is the slope of the tangent line to the curve rr when θ=π3\theta=\frac{\pi}{3} ? Give an exact expression.
slope == \square
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Answer from Sia
Posted 9 months ago
Solution by Steps
step 1
To find the slope of the tangent line to the polar curve r=4sin(3θ)r=4\sin(3\theta) at θ=π3\theta=\frac{\pi}{3}, we first differentiate rr with respect to θ\theta
step 2
The derivative of rr with respect to θ\theta is given by drdθ=ddθ(4sin(3θ))\frac{dr}{d\theta} = \frac{d}{d\theta}(4\sin(3\theta))
step 3
Applying the chain rule, drdθ=43cos(3θ)=12cos(3θ)\frac{dr}{d\theta} = 4 \cdot 3\cos(3\theta) = 12\cos(3\theta)
step 4
Evaluating this derivative at θ=π3\theta=\frac{\pi}{3}, we get drdθθ=π3=12cos(π)=12\frac{dr}{d\theta}\bigg|_{\theta=\frac{\pi}{3}} = 12\cos(\pi) = -12
step 5
The slope of the tangent line in polar coordinates is given by dydx=drdθsin(θ)+rcos(θ)drdθcos(θ)rsin(θ)\frac{dy}{dx} = \frac{\frac{dr}{d\theta}\sin(\theta) + r\cos(\theta)}{\frac{dr}{d\theta}\cos(\theta) - r\sin(\theta)}
step 6
Substituting r=4sin(3θ)r=4\sin(3\theta) and drdθ=12\frac{dr}{d\theta}=-12 into the slope formula, we get dydx=12sin(π3)+4sin(π)12cos(π3)4sin(π)\frac{dy}{dx} = \frac{-12\sin(\frac{\pi}{3}) + 4\sin(\pi)}{-12\cos(\frac{\pi}{3}) - 4\sin(\pi)}
step 7
Simplifying the expression, we find dydx=12321212=636=3\frac{dy}{dx} = \frac{-12\cdot\frac{\sqrt{3}}{2}}{-12\cdot\frac{1}{2}} = \frac{-6\sqrt{3}}{-6} = \sqrt{3}
Answer
3\sqrt{3}
Key Concept
Finding the slope of a tangent line to a polar curve at a given angle
Explanation
The slope of the tangent line to a polar curve at a given angle is found by differentiating the polar equation with respect to θ\theta, evaluating at the given angle, and then using the formula for the slope in polar coordinates.

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