What is the Area Between Two Polar Curves?

In summary, to find the area inside r = 9sinθ but outside r = 2, we can use the formula Area = 1/2(Integral of (f(θ)^2 - g(θ)^2)dθ. By substituting in the equations for f(θ)^2 and g(θ)^2, we get a simplified equation of 36.5θ - sin(2θ)/4. By integrating this equation from the intersection point to π/2 and multiplying by two, we get the full area, which is approximately 114.6681319. However, in order to avoid rounding errors, it is recommended to use the arcsin(2/9) instead of the approximate value for
  • #1
PsychonautQQ
784
10

Homework Statement


Find the area inside r = 9sinθ but outside r = 2




Homework Equations


Area = 1/2(Integral of (f(θ)^2 - g(θ)^2)dθ



The Attempt at a Solution


f(θ)^2 =
81sin^2θ = 81((1-cos(2θ))/2)
g(θ)^2 = 4

f(θ)^2 - g(θ)^2 = 36.5 - cos(2θ)/2
integral of (36.5 - cos(2θ)/2)
[36.5θ - sin(2θ)/4]

Area = 1/2[36.5θ - sin(2θ)/4]
If I integrate from 0 to ∏/2 then multiply that area times two I get 114.6681319
Which is the wrong answer.
Where did I go wrong ;-(
 
Physics news on Phys.org
  • #2
How did you get the limits 0 to π/2? Have you sketched the two circles?
 
  • #3
Should I integrate from the intersection point to pi/2?
9sin(theta) = 2
theta = arcsin(2/9)
theta = .224093
If I do that that is half of the area and then I can multiply that by two to get the full area. Does that look better to you?
 
  • #4
PsychonautQQ said:
Should I integrate from the intersection point to pi/2?
Yes, that is valid because the shaded region is symmetric wrt the y axis.
9sin(theta) = 2
theta = arcsin(2/9)
theta = .224093
I would suggest subbing in arcsin(2/9). The range of this function is [-pi/2,pi/2] so subbing this in will give you the required theta.

If I do that that is half of the area and then I can multiply that by two to get the full area. Does that look better to you?
Yes.
 
  • #5
CAF123 said:
I would suggest subbing in arcsin(2/9). The range of this function is [-pi/2,pi/2] so subbing this in will give you the required theta.


Yes.
I plug these in for theta and then get a required theta? I'm confused by what you mean here. Do you mean that the limits of integration are [-∏/2,∏/2]?
 
  • #6
PsychonautQQ said:
I plug these in for theta and then get a required theta? I'm confused by what you mean here. Do you mean that the limits of integration are [-∏/2,∏/2]?
No, just sub in arcsin(2/9) instead of subbing in 0.224... to avoid rounding errors. The principal solution of arcsin(2/9) is the one that appears in [-pi/2,pi/2] which is the theta that the two curves intersect at in the first quadrant.
You could find the other theta corresponding to the other intersection and integrate between these two values, but your method is also good, perhaps more elegant given you have noticed the symmetry.
 

Related to What is the Area Between Two Polar Curves?

1. What is the formula for finding the area between two polar curves?

The formula for finding the area between two polar curves is given by the integral of 1/2 times the difference between the outer and inner curves, squared, with respect to theta. This can be represented as: A = 1/2 ∫(r_outer^2 - r_inner^2) dθ.

2. How do I determine the limits of integration for finding the area between polar curves?

The limits of integration for finding the area between polar curves can be determined by finding the points of intersection between the two curves. These points will serve as the limits for the integral with respect to theta.

3. Can I use the same technique for finding the area between polar curves as I would for Cartesian curves?

No, the technique for finding the area between polar curves is different from that used for Cartesian curves. This is because polar coordinates use a different coordinate system and require the use of polar equations and integration with respect to theta.

4. Is it possible to find the area between three or more polar curves?

Yes, it is possible to find the area between three or more polar curves by using multiple integrals. The first integral would involve finding the area between the outermost curve and the innermost curve. Then, this result would be used in a second integral to find the area between the next outermost curve and the previous innermost curve, and so on.

5. Can I use polar curves to find the area of irregular shapes?

Yes, polar curves can be used to find the area of irregular shapes. By using multiple polar equations, we can divide the irregular shape into smaller, more manageable areas and then add them together to find the total area of the shape.

Similar threads

  • Calculus and Beyond Homework Help
Replies
3
Views
426
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
Replies
1
Views
869
  • Precalculus Mathematics Homework Help
Replies
7
Views
791
  • Calculus and Beyond Homework Help
Replies
4
Views
2K
  • Calculus and Beyond Homework Help
Replies
4
Views
2K
  • Calculus and Beyond Homework Help
Replies
1
Views
4K
Replies
7
Views
3K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
Back
Top