Semicircular disk
= Semicircular disk
{title2=$\{(x,y):x^2+y^2\leq a^2,\ y\geq0\}$}
A semicircular disk is one half of a <disk> cut along a diameter, including its interior. It is a <circular sector> of angle $\pi$, with <area> $\pi a^2/2$. Source <polar coordinates> use $0\leq r\leq a$, $0\leq\theta\leq\pi$, and <area element> $r\,dr\,d\theta$. Its boundary consists of a <semicircle> and a diameter; the <semicircle> alone is a <curve> and has no planar interior.