A circle with center (h, k) and radius r is every point (x, y) whose distance from the center is r. The Pythagorean theorem turns that into (x - h)² + (y - k)² = r². An expanded equation hides the center and radius until you complete the square. A parabola is every point as far from a fixed point, the focus, as from a fixed line, the directrix; with the vertex at the origin and the focus at (0, p) its equation is y = x²/(4p). This week the crew writes the turntable and the lamp reflector as equations on the stage grid.