Angle (geometry).

Angle (geometry). Angle (geometry), a measure of the rotation of a line segment about a fixed point. This rotation is similar to the sweep of a second hand partway around a clock face, creating a figure consisting of two straight line segments that meet at a point (the center of the clock face). The segments are called the sides or arms of the angle, and the point is called the vertex of the angle. Angles are usually measured in degrees, minutes, and seconds. There are 60 minutes in one degree, and 60 seconds in one minute. Positive rotation is conventionally counterclockwise; negative rotation, clockwise. One complete rotation is 360 degrees. A precisely measured angle of 32 degrees 41 minutes 17 seconds can be compactly expressed as 32°41'17". Angles are also expressed in units called radians, which are expressed as multiples of pi (p). Pi is the ratio of the circumference of a circle to its diameter. 2p radians is equal to 360°. See also Radian. Some angles have special names. An angle of 90° is called a right angle, and its sides are perpendicular to each other. An angle measuring 180° is called a straight angle, because its sides form a straight line. Angles between 0° and 90° are called acute angles, those between 90° and 180° are called obtuse angles, and those between 180° and 360° are called reflex angles. If the sum of two angles is 90°, the angles are said to be complementary; if the sum is 180°, the angles are supplementary; and if the sum is 360°, the two angles are conjugate. Corresponding angles are the pairs of equal angles formed when a single straight line crosses two parallel lines. Adjacent angles are angles that meet at a common vertex (corner) and share a common arm. See Geometry; Trigonometry. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.