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The point where both rays (sides) of an angle meet.

equilateral triangle

A triangle in which all sides are the same length and all angles are the same size.

right triangle

A triangle that has a right angle. (90 degree angle)

obtuse triangle

A triangle in which one angle is larger than 90 degrees.

acute triangle

A triangle in which all three angles are less than 90 degrees.

isosceles triangle

A triangle in which two sides are the same length and the two base angles have the same measure.

scalene triangle

A triangle in which no two sides are of the same length.


The longest side of a right triangle, opposite the right angle.

leg (right triangle)

The two sides of a right triangle that form the right angle.


The third side of an isosceles triangle, it may not be congruent to the other two sides.

leg (isosceles triangle)

The two sides of an isosceles triangle that are congruent.

equiangular triangle

A triangle in which the three angles are congruent.

adjacent sides

Sides of a triangle that share a vertex.

opposite side

The side across from an angle.


A polygon with three sides.

vertex angle

The angle opposite the base of an isosceles triangle.

180 degrees

The sum of the interior angles of a triangle.

base angles

The angles opposite the congruent sides of an isosceles triangle.

congruent triangles

Two triangles that are have the same shape and size.

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