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parallel

coplanar lines that do not intersect; everywhere equidistant

perpendicular

intersecting at 90 degree angles

vertical angle theorem

Vertical angles are congruent

linear pair

two supplementary adjacent angles; opposite rays form a straight line

corresponding angles postulate

if // lines are crossed by a transversal, corresponding angles are congruent

alternate interior angles theorem

if // lines are crossed by a transversal, alternate interior angles are congruent

same side interior angle theorem

if // lines are crossed bya transversal, same side interior angles are supplementary

triangle sum theorem

the sum of the angles in a triangle is 180

Isosceles triangle theorem

the base angles of an isosceles triangle are congruent

reflexive property

AB = AB

transitive property

if A = B and B =C, then A = C

symmetric property

AB = BA

converse of Isosceles triangle theorem

If 2 angles in a triangle are congruent, then the sides opposite those angles are also congruent

180(n-2)

Sum of the degrees of a polygon with 'n' sides

angle bisector

line, segment, or ray that splits an angle into two equal angles

perpendicular bisector

line, segment or ray that intersects a segment at its midpoint at a 90 degree angle

midpoint

point on a segment exactly halfway between the endpoints

HL theorem

If the hypotenuse and one of the legs of a right triangle are congruent to the hypotenuse and one of the legs of another right triangle, then the triangles are congruent

SSS

If 3 sides of one triangle are congruent to 3 sides of another, then the triangles are congruent

SAS

If two sides and the INCLUDED angle of one triangle are congruent to two sides and the INCLUDED angle of another, then the triangles are congruent

ASA

If two angles and the included side of one triangle are congruent to two angles and the included side of another triangle, then the triangles are congruent

AAS

If any two angles and one side of a triangle are congruent to two angles and a side of another triangle, then the triangles are congruent

vertex of isosceles triangle

angle formed by the two congruent legs

base angles

angles opposite the two congruent legs of an isosceles triangle

hypotenuse

longest side of a right triangle; opposite the right angle