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## Angles and Their Measure

### Video Summary:

Basic Terminology Two distinct points A and B determine the line AB. The portion of the line including the points A and B is the line segment AB. The portion of the line that starts at A and continues through B is called ray AB. An angle is formed by rotating a ray, the initial side, around its endpoint, the vertex, to a terminal side. Degree Measure Developed by the Babylonians around 4000 yrs ago. Divided the circumference of the circle into 360 parts. One possible reason for this is because there are approximately that number of days in a year. There are 360° in one rotation. An acute angle is an angle between 0° and 90°. A right angle is an angle that is exactly 90°. An obtuse angle is an angle that is greater than 90° but less than 180°. A straight angle is an angle that is exactly 180°. If the sum of two positive angles is 90°, the angles are called complementary. If the sum of two positive angles is 180°, the angles are called supplementary. Quadrantal Angles are angles in standard position (vertex at the origin and initial side along the positive x- axis) with terminal sides along the x or y axis, i.e. 90°, 180°, 270°, etc. Coterminal Angles are angles that have the same initial side and the same terminal side.