Adjacent Angles
Two angles that share a common vertex and a common ray (side), but that do not share any interior points.
For example, in the diagram below, ?BAC and ?CAD share vertex A and side , but no interior points; they form a pair of adjacent angles.
?BAC and ?BAD
Vertical Angle
A pair of nonadjacent angles formed by the intersection of two lines. Vertical angles have equal measure. The diagram below shows a pair of vertical angles.
Supplementary Angles
Two angles are supplementary if the sum of their measures equals 180�.
In the diagram below, ?1 and ?2 are supplementary.
Complementary Angles
Two angles are complementary if their measures add up to 90�. In the diagram, ?ADB and ?BDC are complementary.
Midpoint
The midpoint, C, of line segment is a point lying on segment that divides into two segments of equal length, that is, AC = CB.
Bisect
To divide into two equal portions.
Altitude
A line segment drawn from one point or side in a plane figure that meets another side at a right angle.
Altitudes are often constructed in triangles, parallelograms, and trapezoids, because the length of the altitude (height) is used in computing the area
Median of a Triangle
A line segment drawn from one vertex of a triangle to the midpoint of the opposite side. In the figure below, is a median of ?ABC:
Perpendicular Lines
A pair of lines that intersect, forming a right angle, usually indicated by a small square, as shown in the diagram below. "Line AB is perpendicular to line CD" is expressed as: ? .
Linear pair of angles
A pair of angles that are both adjacent and supplementary.
In the diagram, ?ABD and ?DBC are a linear pair of angles.
Reflexive Property
A quantity is congruent (equal) to itself. a = a
Symmetric Property
If a = b, then b = a.
Transitive Property
If a = b and b = c, then a = c.
Addition Postulate
If equal quantities are added to equal quantities, the sums are equal. (If AB = CD and EF = EF, then AB+EF = CD+EF)
Subtraction Postulate
If equal quantities are subtracted from equal quantities, the differences are equal.