An angle is defined as the amount of rotation between two rays. There are two systems for measuring angles in mathematics: degrees and radians. A full rotation in degrees is . A full rotation in radians is (tau) radians or approximately radians. For higher level math, radians are the preferred system to measure angles.
An acute angle is an angle that is smaller than 90 degrees or PI fourths.
An obtuse angle is an angle that is larger than 90 degrees or PI fourths.
A Perpendicular angle, sometimes also referred to as a square angle, is exactly 90 degrees or PI fourths.
Complementary angles can visually be denoted as two angles who sum to a perpendicular or square angle.
Supplementary angles can visually be denoted as two angles who sum to 180 degrees or PI degrees.
The degree angle system divides a full rotation into degrees.
Radians Versus DegreesWumbo (internal)