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AAS TheoremIf two corresponding angles and one of the sides not between the angles in any two triangles are congruent, the two triangles are congruent.
Acute TriangleA triangle that's slightly less cute than a-super-cute triangle. For that to be true, all the angles in the triangle have to be less than 90°.
Angle Sum TheoremAll the interior angles of a triangle add up to 180°. Always.
ASA PostulateIf two corresponding angles and the included side in any two triangles are congruent, the two triangles are congruent.
CongruenceWhen two objects are identical in every way. The only thing that separates them is a congruence transformation.
CPCTCStands for "Corresponding Parts of Congruent Triangles are Congruent." The "parts" we're talking about here are angles and sides, so don't go snooping at the local auto body shop.
Equilateral TriangleA triangle that has three congruent sides. It should also be known that an equilateral triangle has three 60° congruent angles as well.
Exterior AngleAn angle supplementary to one of the triangle's inner angles. Just extend one of the sides of the triangle, and you'll see what we mean.
Exterior Angle TheoremThe measure of one exterior angle is equal to the measure of the remote interior angles (the inner angles that aren't supplementary to the exterior angle).
Hypotenuse-Leg TheoremIf the hypotenuses and corresponding legs of two triangles are congruent, the two triangles are congruent.
Interior AngleOne of the inside angles in a triangle. They're innies, not outies.
Isosceles TriangleA triangle with at least two congruent sides (or, from the Isosceles Triangle Theorem, two congruent angles).
Isosceles Triangle TheoremTwo sides of a triangle are congruent to each other only if the angles opposite the sides are congruent to each other. The sides and angles can go on double-dates.
Obtuse TriangleA triangle that's a little slow on the uptake. Or one with an angle that's over 90°.
Remote Interior AnglesTwo of a triangle's interior angles that aren't supplementary to a given exterior angle.
Right TriangleA triangle that has an angle that's exactly 90°. Or possibly a triangle that's just never wrong about anything.
Scalene TriangleA triangle whose three sides are all different lengths. He's probably just going through a growth spurt.
SAS PostulateIf two corresponding sides and one of the angles between the sides in any two triangles are congruent, the two triangles are congruent.
SSS PostulateIf all three sides in any two triangles are congruent, the two triangles are congruent. This is what gives triangles their rigid structure (but it won't make them any more emotionally stable).