= Circular segment
{title2=$A=\tfrac12R^2(\phi-\sin\phi)$}
A <circular segment> is the part of a disk bounded by a chord and one of the two <circular arcs> joining its endpoints. For <radius> $R$ and the chosen arc's central <angle> $0\leq\phi\leq2\pi$ in <radians>, its <area> is $R^2(\phi-\sin\phi)/2$. Subtract the signed central <triangle> of area $R^2\sin\phi/2$ from the <circular sector> of area $R^2\phi/2$. Angles below $\pi$ give the smaller segment and angles above $\pi$ the larger one.
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