You have worked with various types of triangles in past courses.
Let's refresh some basic facts we know to be true about triangle types.

Triangles Classified by Sides
A scalene triangle has three sides of different lengths.
No congruent sides.
Note: The angles in a scalene triangle are also of different measures.

An isosceles triangle has two congruent sides.
The sides that are the same lengths are called the "legs".
The other side is called the "base".
The angle between the legs is called the "vertex angle".
The angles containing the base as a side are called the "base angles".
Note: The base angles in an isosceles triangle are of the same measure.

An equilateral triangle has three congruent sides.
An equilateral triangle may also be called a "regular" triangle.
(A "regular" polygon has all equal sides and all equal angles.)
Note: The angles in an equilateral triangle are also of equal measures (60º each).


Triangles Classified by Angles
An acute triangle has all angles measuring less than 90º.
Note: It is possible for an acute triangle to also be scalene, isosceles, or equilateral.

A right triangle has one right angle.
(A right angle measures exactly 90º.)
A "box" is used to indicate the location of the right angle.
The longest side of the right triangle (across from the "box") is called the "hypotenuse".
The remaining two sides are referred to as "legs", which may, or may not, be of equal length.
Note: It is possible for a right triangle to also be scalene or isosceles.


An obtuse triangle has one angle measuring more than 90º but less than 180º (an obtuse angle).
It is not possible to draw a triangle with more than one obtuse angle.
Note: It is possible for an obtuse triangle to also be scalene or isosceles.

An equiangular triangle has three congruent angles.
Each angle has a measure of 60º.
Note: All equiangular triangles are also equilateral.


An oblique triangle is any triangle that is not a right triangle.
It may be acute, obtuse, equiangular, scalene, isosceles, or equilateral, but not a right triangle.

Oblique triangle


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