Central angle intersection points and of the legs of the angle with the circle form a diameter, then is a straight angle. (In radians, .) Let be the minor arc of the circle between points and , and let be the radius of the circle. If the central angle is subtended by , then If the central angle is not subtended by the minor arc , then is a reflex angle and If a tangent at and a tangent at intersect at the exterior point , then denoting the center as , the angles (convex) and are supplementary (sum to 180°). A
Straight razor as heavy stropping and honing. Shaving is done with the blade at approximately an angle of thirty degrees to the skin and in a direction perpendicular to the edge; an incision requires the movement of the blade to be sideways or in a direction parallel to the edge. These circumstances are always avoided by the shaver, who always shaves in a direction perpendicular to the cutting edge of the blade. To be most effective, a straight razor must be kept extremely sharp. The edge is delicate, and inexpert use may bend or fold over the razor's edge. To unfold and