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Sketch two circles tangent to the y-axis and compute their area from 0 to 2 π

by RoRi

Define two circles tangent to the y-axis by:

    \[ f(\theta) = 2 |\cos \theta|, \qquad 0 \leq \theta \leq 2 \pi. \]

Sketch this graph in polar coordinates and compute the area of the radial set.

The sketch is as follows:

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Then, we compute the area,

    \begin{align*} a(R) = \frac{1}{2} \int_0^{2 \pi} 4 \cos^2 \theta \, d \theta &= 2\int_0^{2 \pi} \left( \frac{1 + \cos (2\theta)}{2} \right) \, d \theta \\ &= \int_0^{2 \pi} d \theta + \int_0^{2 \pi} \cos (2 \theta) \, d \theta \\ &= 2 \pi + 0 = 2 \pi. \end{align*}

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