Find the Exact length of the Polar Curve

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Homework Help Overview

The discussion revolves around finding the exact length of the polar curve defined by the equation r=2(1+cosθ). Participants note the absence of limits of integration and explore the implications of this in the context of the problem.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the setup of the problem, including the equation for the length of the curve and the derivative of r with respect to θ. There are attempts to derive the integral for the length, and some participants express uncertainty about the correctness of their expressions and the limits of integration.

Discussion Status

Some participants have provided guidance on determining the limits of integration by evaluating the polar curve at specific angles. Others have suggested methods for integrating the resulting expression, including substitution techniques. There is ongoing exploration of the implications of the integral's evaluation over the specified range.

Contextual Notes

Participants note the lack of explicit limits of integration and question whether the integral setup is correct. The discussion includes references to the shape of the curve and its periodic nature, which may influence the limits chosen for integration.

CitizenInsane
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Homework Statement


Find the Exact length of the Polar Curve for
r=2(1+cosθ)

No limits of Integration were given which I found to be odd.

Homework Equations



L= ∫√(r^2+(dr/dθ)^2)dθ

The Attempt at a Solution



r=2(1+cosθ)
dr/dθ=-2sinθ

L=∫√((2+2cosθ)^2+(-2sinθ)^2)dθ
=∫√(4cos^2θ+4sin^2θ+8cosθ+4)dθ
=∫√(8cosθ+8)dθ

Got to this point and figured I did something incorrectly.
 
Last edited:
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CitizenInsane said:

Homework Statement


r=2(1+cosθ)
It's not a good idea to put the statement of the problem only in the title! I started to say "no that is not a problem statement" until I looked up at your title!

No limits of Integration were given which I found to be odd.
Perhaps they expect you to be able to get that yourself. When \theta= 0, cos(\theta)= 1 so r= 4; when \theta= \pi/2, cos(\theta)= 0 so r= 2; when \theta= pi, cos(\theta)= -1 so r= 0; when \theta= 3\pi/2, cos(\theta)= 0 so r= 2 and when \theta= 2\pi, cos(\theta)= 1 so r= 4 again. We make one complete loop around the figure (a "cardiod" since it looks roughly like the "valentine" heart) as \theta goes from 0 to 2\pi. That shouldn't be too surprizing since cosine has period 2\pi.

Homework Equations



L= ∫√(r^2+(dr/dθ)^2)dθ

The Attempt at a Solution



r=2(1+cosθ)
dr/dθ=-2sinθ

L=∫√((2+2cosθ)^2+(-2sinθ)^2)dθ
=∫√(4cos^2θ+4sin^2θ+8cosθ+4)dθ
=∫√(8cosθ+8)dθ

Got to this point and figured I did something incorrectly.
No, that's correct. Now integrate from 0 to 2\pi.
 
Like HallsofIvy said, you should plot the polar curve: r=2(1+cosθ) to find the limits of integration. I have attached the graph to this post.
 

Attachments

  • polar.gif
    polar.gif
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Sorry for not putting enough info originally, new to the forums.
Also when I integrate, I get stuck at
=√8∫√(cosθ+1)dθ
Only step I could think of next is to u-sub what's inside the radical.
 
CitizenInsane said:
Sorry for not putting enough info originally, new to the forums.
Also when I integrate, I get stuck at
=√8∫√(cosθ+1)dθ
Only step I could think of next is to u-sub what's inside the radical.

Try trigonometric substitution. Let \sqrt{\cos \theta}=\tan \phi

After integrating, the answer should be: 2\sqrt{8}.\sqrt{\cos \theta +1}.\tan \frac{\theta}{2} Now, you have to put the limits and evaluate.
The problem is that for 0 \leq \theta \leq 2\pi, the evaluation gives 0.

Are you sure the integral for L is correct in your first post?
 
Last edited:

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