Basic Geometry

Angles greater than 0° , but less than 90° are called acute angles.

 

Angles greater than 90° , but less than 180° are called obtuse angles.

Angles equal to 90° are right angles.

Angles equal to 180° are called straight angles.

Angles greater than 180° , but less that 360° are called reflex angles.

Two adjacent angles that add up to 90° are called complementary angles.

Two adjacent angles that add up to 180° are called supplementary angles.

Multiple angles on a line always add up to 180° .

Multiple angles at a point always add up to 360° .

Parallel Lines

Lines are parallel if..

Alternate Interior angles are equal. (Z-rule)

Corresponding angles are equal. (F-rule)

Co-Interior angles are supplementary. (C-rule)

Vertically Opposite angles are equal. (X-rule)

Midpoint

The midpoint of a line segment is the point that is halfway between the endpoints of the line segment.

Median

The median of a triangle is the segment from a vertex to the midpoint of the opposite side.

Altitude

The altitude of a triangle is a line segment from one vertex of a triangle to the opposite side so that the line segment is perpendicular to the side.

Angle Bisector

The angle bisector is a line that divides an angle into two parts.

Perpendicular Bisector

The perpendicular bisector is a line which cuts a line segment into two parts at 90°.