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### Course: AP®︎ Calculus BC (2017 edition)>Unit 4

Lesson 9: Polar functions differentiation

# Worked example: differentiating polar functions

An AP Calculus sample item where we find the rate of change of 𝘹 with respect to θ.

## Want to join the conversation?

• At , Sal says the function 'flips' it over into positive. Why is this? Why can't the function r be negative?
(27 votes)
• I had the same question. This helped:

https://youtu.be/ii31Gy4quqo?list=PLX2gX-ftPVXXWNn8FQ8DfZ0N0YVJCxe-p

https://youtu.be/DqfG8jLyfzQ?t=446

Lets's evaluate a simpler function at a few points.

r(a) = 4 sin (a)
--------------
r(pi/2) = 4sin(pi/2) = 4*1 = 4
r(pi) = 4sin(pi) = 4*0 = 0
r(1.5pi) = 4sin(1.5pi) = 4*-1 = -4

The insight is that you look at the sign of the radius. Here it is negative. So you prepare to put the point where you expect it to, but you can't put it there because that would mean the radius was positive after all, that would be the point (4, 1.5pi). To plot (-4, 1.5pi), make the arrow of the radius point in the opposite direction, up instead of down.
(22 votes)
• Why is zeta pi/2 at ?
(11 votes)
• Theta is pi/2 at point P on the graph, which sits on the y axis. As you go around the unit circle, the axes represent certain divisions on the unit circle, either in radians (in this case) or in degrees. Here, we've gone one quarter of the way around the unit circle, which is made up of 2pi radians. A quarter of 2pi is pi/2. This is also the angle of theta at point P.
(10 votes)
• I dont' get how the initial function can draw the figure. polar coordinates has 2 coordinates, angle and radius, the function only has one.
(2 votes)
• It actually has both. r is the radius, so the length and direction of the radius is defined by 3(theta)sin(theta).
(2 votes)
• at , θ=pi/2. btw, how to judge what the θ is? i.e., in the practice question, θ is pi/4, but the corresponding graph of r is in the 3rd quadrant.
(2 votes)
• Usually, you'd want to take some value of theta, sweep that angle, plug it in to find the value of r, and move that many units in the direction you already swept. So, if you plug in π/2 into the polar equation, you get r = 4.71. So, you first sweep out π/2 radians, then move 4.71 units along the direction swept (which happens to be the positive y axis).

Would you happen to know the polar equation of that problem? Polar graphs do give some unexpected results sometimes, but it's hard to determine what happened without the equation.
(2 votes)
• Is there a way to see the rate of change of x on the graph? Like how you can find a tangent line when you take the derivative of an equation
(2 votes)
• To find the derivative of y with respect to x, find the derivative of y with respect to theta and divide that by the derivative of x with respect to theta.

dy/dx = (dy/dθ)/(dx/dθ) = y’(θ)/x’(θ)

This can be used to find the slope of the tangent line.
(1 vote)
• How do we know the smaller loop happens first? Couldn't it loop around the bigger loop then flip to do the smaller one?
(2 votes)
• at , when going from pi to 2pi, sinθ is negative, and then r (radius) is a negative number?
(1 vote)
• Correct. However, r is defined to be non-negative. Hence, Sal says that it flips it over.
(2 votes)
• Is there a video that explains the second derivative clearly?
(1 vote)
• Can Khan Academy upload a video on how specifically to do the types of problems in the Tangents to Polar Curves quiz?
(1 vote)

## Video transcript

- [Instructor] Let r be the function given by r of theta is equal to three theta sine theta for theta is between zero and two pie, including zero and two pie. The graph of r in polar coordinates consists of two loops, as shown in the figure above. So let's think about why it has two loops. So as our theta, when theta is zero, r is zero, and then as our theta increases, we start tracing out this first loop, all the way until when theta's equal to pie. So we've traced out this first loop from theta's equal to zero to theta's equal to pie. And then the second loop has a larger r, so these are larger r's, this is when we're going from pie to two pie. And you might say, "Well why doesn't it show up down here?" Well, between sine of pie and sine of two pie, this part right over here is going to be negative, so it flips it over, the r onto this side. And the magnitude of the r is larger and larger because of this three theta. And so when we go from pie to two pie, we trace out the larger circle, fair enough, that seems pretty straightforward. Point P is on the graph of r, right over there, and the y-axis. Find the rate of change of the x-coordinate with respect to theta at the point P. Alright, so let's think about this a little bit. They don't give us x as a function of theta. We have to figure out that from what they've given us. So just as a bit of a polar coordinates refresher, if this is our theta, right over there, this is our r, and that would be a point on our curve for this theta. Now how do you convert that to x and y's? Well you can construct a little bit of a right triangle right over here. And we know from our basic trigonometry that the length of this base right over here, this is going to be the hypotenuse. Let me just write that, that's going to be our x-coordinate. Our x-coordinate right over here is going to be equal to our hypotenuse which is r times the cosine of theta. If we wanted the y-coordinate as a function of r and theta, it'd be y is equal to r sine of theta, but they don't want us to worry about y here, just the x-coordinate. So we know this, but we want it purely in terms of theta. So how do we get there? Well what we can do is, take this expression for r, r itself is a function of theta, and replace it right over there. And so what we can do, is we can write, well, x of theta is going to be equal to r, which itself is three theta sine of theta times cosine of theta, times cosine of theta. And now, we wanna find the rate of change of the x-coordinate with respect to theta at a point, so let's just find the derivative of x with respect to theta. So x prime of theta is equal to, well I have the product of three expressions over here. I have this first expression, three theta, then I have sine theta, and then I have cosine theta. So we can apply the product rule to find the derivative. If you're using the product rule with the expression of three things, you essentially just follow the same pattern when you're taking the product of two things. The first term is gonna be the derivative of the first of the expressions, three, times the other two expressions, so we're gonna have three times sine of theta cosine of theta, plus the second term is going to be the derivative of the middle term times the other two expressions, so we're gonna have three theta and then derivative of sine theta is cosine theta, times another cosine theta, you're gonna have cosine squared of theta, or cosine of theta squared, just like that. And then you're gonna have the derivative of the last term, is going to be the derivative of cosine theta times these other two expressions. Well the derivative of cosine theta is negative sine theta, so if you multiply negative sine theta times three theta sine theta, you're going to have negative three theta sine squared theta. And so, we want to evaluate this at point P. So what is theta at point P? Well, point P does happen on our first pass around, and so, at point P, theta is equal to theta right over here is equal to pie over two, so pie over two, so what we really just need to find is, well what is x prime of pie over two? Well, that is going to be equal to three times sine of pie over two, which is one, times cosine of pie over two, which is zero, so this whole thing is zero, plus three times pie over two, this is three pie over two, times cosine squared of pie over two, or cosine of pie over two squared. Well that's just zero, so so far, everything is zero, minus three times pie over two, three pie over two times sine of pie over two squared. Well what's sine of pie over two? Well that's one, you square it, you still get one. So all of this simplified to negative three pie over two. Now it's always good to get a reality check. Does this make sense that the rate of change of x with respect to theta is negative three pie over two? Well think about what's happening. As theta increases a little bit, x is definitely going to decrease, so it makes sense that we have a negative out here. So, right over here, rate of change of x with respect to theta, negative three pie over two. As theta increases, our x for sure is decreasing, so at least it does make intuitive sense.