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Acute Triangle
3 acute angles

Equiangular Triangle
Three congruent acute angles

Right Triangle
One right angle

Equilateral Triangle
Three congruent sides

Scalene Triangle
No congruent sides

Interior Angle
formed by two sides of a triangle

Exterior angle
formed by one side of the triangle and the extension of an adjacent side. Each exterior angle has two remote interior angles

Remote interior angle
interior angle that is not adjacent to the exterior angle

Corresponding angles + Corresponding sides
the same position in polygons with an equal number of sides

Congruent polygons
(example) Two polygons are congruent polygons if and only if their corresponding angles and sides are congruent. Thus triangles that are the same size and shape are congruent

Triangle rigidity
provides a shortcut for proving two triangles congruent. (example) It states that if the side lengths of a triangle are given, the triangle can only have one shape

Included angle
Angle formed by two adjacent sides of a polygon

Included side
Common side of two consecutive angles in a polygon

AngleSideAngle (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

HypotenuseLeg (HL)
If the hypotenuse and a leg of a right triangle are congruent to the hypotenuse and a leg of another right triangle, then the triangles are congruent

CPCTC  abbreviation for the phrase “Corresponding Parts of Congruent Triangles are Congruent.”
Can be used as a justification in a proof after you have proven two triangles congruent

Coordinate proof
A style of proof that uses coordinate geometry and algebra. The first step of a coordinate proof is to position the given figure in the plane

Legs
shorter sides of a angle or triangle

Vertex angle
angle formed by the legs

Base
opposite side of the vertex angle

Base angles
two angles that have the base as a side

