# Polar equation problems

## Homework Statement

$$r = \theta + sin(2\theta)$$ for $$0 \le \theta \le \pi$$

a. Find the area bounded by the curve and the x-axis

b. Find the angle $$\theta$$ that corresponds to the point on the curve with x-coordinate -2

c. For $$\frac{\pi}{3} < \theta <\frac{2\pi}{3} , \frac{dr}{d\theta}$$ is negative. What does this fact say about r? What does this fact say about the curve?

d. Find the value of $$\theta$$ in the interval $$0 \le \theta \le \frac{\pi}{2}$$ that corresponds to the point on the curve in the first quadrant with the greatest distance from the origin.

## The Attempt at a Solution

a. $$1/2\int_{0}^{\pi }(\theta + sin2\theta)^2 d\theta}$$

When I foil I end up with $$\theta^2 + 2\theta\sin2\theta + sin^2(2\theta)$$

this is the part where I believe I made the mistake. I know the area is 4.9348 by the use of my calculator...if it is indeed correct...I can easily integrate the 1st and 3rd term...but i think the second term needs to be done by parts if im not mistaken.

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Parts should be ok to solve that. It's pretty straight forward. If you're familiar with the "ultra-violet voodoo" pneumonic, then let the $$\theta$$ term be the "doo" (du) part.

i seem to be having trouble doing the integration by parts

$$2\theta\sin2\theta$$

using uv - $$\int$$ vdu

u = 2$$\theta$$ dv = $$sin2\theta$$
du = 2$$d\theta$$ v = $$\frac{-1}{2}cos2\theta$$

$$-\theta\cos2\theta + 1/2sin2\theta$$

the final integral from $$\theta^2 + 2\theta\sin2\theta + sin^2(2\theta)$$ becomes:

$$\frac{1}{3}\theta^3-\theta\cos2\theta+ 1/2sin2\theta + \frac{1 - cos4\theta}{2}$$

from 0 to $$\pi$$

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Try

$$u=2\theta$$
$$\Rightarrow du=2d\theta$$
$$v=\sin 2\theta$$
$$\Rightarrow dv=-2\cos 2\theta$$

Then plug into your expression of

$$uv-\int v du$$

Don't forget to evaluate that first part with limits.

Last edited:
Try

$$u=2\theta$$
$$\Rightarrow du=2d\theta$$
$$v=\sin 2\theta$$
$$\Rightarrow dv=-2\cos 2\theta$$

Then plug into your expression of

$$uv-\int v du$$

Don't forget to evaluate that first part with limits.

i'm pretty sure dv = sin2$$\theta$$

and its integral v = -1/2cos2$$\theta$$

Yes, you're correct. But it is of no consequence because we don't need the differential of v.

Wait. Hang on. No.

$$dv=-(1/2)\cos 2\theta d\theta$$

Wait. Hang on. No.

$$dv=-(1/2)\cos 2\theta d\theta$$

you set u to one part, and dv to another

du is the differential of u,

v is the integral of dv.

I'll work on this tomorrow...ahhhh so frustrating.

any help at integrating this?

$$2\theta\sin2\theta$$