What is the area inside the lemniscate and outside the circle?

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In summary, the area inside the lemniscate r^2=150cos(2theta) and outside the circle r=5sqrt(3) is equal to 75 times the difference between the square root of 3 and pi divided by 3. This is found by using symmetry arguments and a well-known formula to calculate the area on the right half plane.
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Fernando Revilla
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I quote a question from Yahoo! Answers

Find the area inside the lemniscate r^2=150cos(2theta) and outside the circle r=5sqrt(3).

I have given a link to the topic there so the OP can see my response.
 
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By symmetry arguments, the area is $A=2A_1$ where $A_1$ is the corresponding area on the right half plane. Equating modules, $$150\cos 2\theta=(4\sqrt{3})^2\Leftrightarrow 150\cos 2\theta=75 \Leftrightarrow \cos 2\theta=\frac{1}{2}\Leftrightarrow \theta=\pm \frac{\pi}{6}$$ Using a well known formula: $$A=2A_1=2\cdot\frac{1}{2}\int_{-\pi/6}^{\pi/6}\left(r_2^2-r_1^2\right)d\theta=\int_{-\pi/6}^{\pi/6}\left(150\cos 2\theta-75\right)d\theta\\=\left[75\sin 2\theta-75\theta\right]_{-\pi/6}^{\pi/6}=75\left(\sqrt{3}-\frac{\pi}{3}\right)$$
 

FAQ: What is the area inside the lemniscate and outside the circle?

1. What is the area inside a lemniscate?

The area inside a lemniscate is the total space enclosed by the curve of the lemniscate. It is a mathematical concept that represents the infinity symbol (∞) and has a symmetric shape.

2. How is the area inside a lemniscate calculated?

The area inside a lemniscate can be calculated using the formula A = 2a², where a is the length of the semi-major axis. This formula is derived from the equation of the lemniscate (x² + y² = a²).

3. What is the significance of the area inside a lemniscate?

The area inside a lemniscate has many applications in mathematics and physics. It is used to calculate the area of certain geometric shapes, as well as in the study of fluid dynamics, electromagnetism, and other fields.

4. Can the area inside a lemniscate be negative?

No, the area inside a lemniscate cannot be negative. The lemniscate is a closed curve, meaning it does not have any gaps or holes, so the area enclosed by it will always be a positive value.

5. How does the area inside a lemniscate change with different values of a?

The area inside a lemniscate is directly proportional to the square of the length of the semi-major axis. This means that as the value of a increases, the area inside the lemniscate also increases. Similarly, as a decreases, the area inside the lemniscate decreases.

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