Basic Concepts of Geometry

The two lines that make an angle are called rays. Theoretically, rays are of infinite length. Angles are measured in degrees; each degree is 1/360 of a round trip (circle).

Acute Angle

Angles greater than 0 degrees and less than 90 degrees are called acute.

Right Angle

Angles that are exactly 90 degrees are called right angles. Right angles are usually represented by a box in the corner of the angle.

Obtuse Angle

Angles that are greater 90 degrees and less than 180 degrees are called obtuse angles.

Reflective Angle

Angles that are greater than 180 degrees and less than 360 degrees are called reflective angles.

Complementary Angle

Angles that add up to 90 degrees are called complementary angles. If x + Y = 90° then x and y are complementary angles.

Supplementary Angles

Angles that add up to 180 degrees are called supplementary angles. If x + y = 180° then x and y are supplementary angles.

Vertical Angles

Angles that are opposite to one another are called vertical angles and are equal.

Circle Angle

All angles around a point add up to 360 degrees.

Alternate Angles

With two parallel lines (Alternate Angles), angles that are in the same position are equal. For instance, in the figure, angles 1 and 5 are both in the upper right – hand corner and must be equal. Angles 3 and 7 are both in the lower left – hand corner and must be equal. Since angle 1 is opposite angle 3 they are equal; angle 5 is opposite angle 7 and they are equal. Therefore, angles 1, 3, 5 and 7 are equal. Also, angles 2, 4, 6 and 8 are equal