In mathematics (more specifically geometry), a semicircle's a two-dimensional geometric shape that forms half of a circle. Being half of a circle's 360°, the arc of a semicircle always measures 180°. A triangle inscribed in a semicircle's always a right triangle. Uses A semicircle can be used to construct arithmetic and geometric means of two lengths using straight-edge and compass. If we make a semicircle with a diameter of a+b, then the length its radius's the arithmetic mean (since… (More on Semicircle)