Theorems, Definitions, Postulates

Bisector

A line, ray, or segment that passes through a segment at its midpoint.

Transversal

A line that intersects two or more coplanar lines at different points.

Same-Side Interior Angles

If two parallel lines are cut by a transversal, then the same side interior angles are supplementary (180 degrees).

Right Angle

Measures to 90 degrees.

Adjacent Angles

Two angles that share a common side and a common vertex but no common interior points.

Congruent Segments

Segments that have equal measure.

Perpendicular Lines and Parallel Lines

Perpendicular Lines (two lines that intersect at 90 degrees)

Parallel Lines (two or more lines that never intersect)

Alternate Interior Angles

Nonadjacent interior angles that lie on the opposite sides of a transversal.

Midpoint

A point that divides a segment into two congruent segments.

Supplementary Angles

Angles that have a sum of 180 degrees.

Perpendicular Bisector

A line, ray, or segment that passes through the midpoint of a segment and is perpendicular to the segment.

Angle Bisector

A ray that divides an angle into two congruent angles.

Corresponding Angles

Angles that lie on the same position on both sides at each intersection.

REMEMBER: if the lines are parallel, then the corresponding angles are equal.

Alternate Exterior Angles

Nonadjacent exterior angles that lie on the opposite sides of a transversal.

Linear Pair

Two adjacent angles whose noncommon sides form a straight angle (180 degrees). 

Vertical Angles

Non-adjacent angles formed by two intersecting lines (in the form of an X-shape).

Complementary Angles

Angles that have a sum of 90 degrees.