Vertex: A vertex is a point where two or more sides of a triangle meet. A triangle has three vertices.
Side: A side is a line segment that connects two vertices of a triangle. A triangle has three sides.
Angle: An angle is formed by two sides of a triangle that meet at a vertex. A triangle has three angles.
Base: The base of a triangle is any side of the triangle that can be used as a reference when finding the height of the triangle.
Height: The height of a triangle is the distance from a vertex to the opposite side, measured at a right angle.
Acute triangle: A triangle is acute if all three of its angles are acute, meaning they are less than 90 degrees.
Obtuse triangle: A triangle is obtuse if one of its angles is obtuse, meaning it is greater than 90 degrees.
Right triangle: A triangle is right if one of its angles is a right angle, meaning it is exactly 90 degrees.
Isosceles triangle: An isosceles triangle is a triangle with two equal sides and two equal angles opposite those sides.
Equilateral triangle: An equilateral triangle is a triangle with three equal sides and three equal angles.
Scalene triangle: A scalene triangle is a type of triangle in which all three sides have different lengths. In other words, none of the sides are equal in length. Also, none of the angles in a scalene triangle are equal in measure, which means that each angle has a different degree measurement.
Exterior Angle Theorem: It states that an exterior angle of a triangle is equal to the sum of its two remote interior angles.
In other words, if we extend one of the sides of a triangle to form an exterior angle, then this exterior angle is equal to the sum of the two non-adjacent interior angles of the triangle.
Triangle Inequality Theorem: It states that the sum of the lengths of any two sides of a triangle is always greater than the length of the third side. In other words, if we have a triangle with sides of lengths and then:
a + b > c
b + c > a
a + c > b
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