An angle is a fundamental concept in geometry. It is formed when two rays meet at a common point called the vertex. The two rays are called the sides or arms of the angle. The angle measures the amount of rotation between the two rays.
Angles are classified into different types based on their measure. An acute angle is less than 90 degrees. A right angle is exactly 90 degrees, forming a perfect corner. An obtuse angle is between 90 and 180 degrees. A straight angle is exactly 180 degrees, forming a straight line.
Angles are measured in degrees or radians. A complete rotation around a point is 360 degrees or 2 pi radians. We use tools like protractors to measure angles accurately. Watch as this angle increases from 0 to 90 degrees, showing how we can measure the rotation between two rays.
角之间可以有特殊的关系。余角是指两个角的和等于90度，比如60度和30度。补角是指两个角的和等于180度，比如120度和60度。当两条直线相交时，会形成对顶角，对顶角总是相等的。