Calculus How To

Leminiscate: Definition, Finding Area

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A leminiscate is a shape traditionally drawn with polar coordinates.
leminiscate

The equation for the leminiscate is r2 = a cos 2θ or r2 = a sin 2θ. Changing a will shrink or expand the shape (to try out different values, check out the interactive leminiscate page at Desmos.com).

Alternatively, the shape can be given as a set of points in a Cartesian plane so that (Piene, 2007)

  • x = (sin t) / (1 + cos2t) and
  • y = (sin t cos t) / (1 + cos2t).

Area of a Leminiscate

You can find the area inside a loop with integrals.

Example question: Find the area inside one loop of a leminiscate r2 = 4 cos 2θ.
Solution:
Step 1: Find the limits of integration. Perhaps the easiest way to do this is to graph one loop. Note that instead of graphing y2 = 4 cos 2θ (which is pretty impossible with most graphing calculators), you can graph y = ±2√(cos 2x) instead:
graph of pm

Using the trace function, we can see that one loop is traced out for θ in the interval
−π/4 < θ < π/4 (or 3π/4 < x < 5π/4).

Step 2: Integrate the function r2 = 4 cos 2θ, using the limits of integration found in Step 1. We’re only finding the area for a single loop here, so the integral needs to be halved:
integrating leminiscate

The area for one loop is 2.

References

Graph of the leminiscate: Desmos.com.
Piene, R. (2007). Curves and Surfaces.
White, D. (2014). Quiz 9A, Math 1860-021 Solutions. Retrieved December 3, 2020 from: http://www.math.utoledo.edu/~dwhite1/d_1860/q9a1860s1.pdf

CITE THIS AS:
Stephanie Glen. "Leminiscate: Definition, Finding Area" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/leminiscate-definition-finding-area/
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