Types of Triangles
Definitions and formulas for triangles including right triangles, equilateral triangles, isosceles triangles, scalene triangles, obtuse triangles and acute triangles
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right triangle  equilateral triangle  isosceles triangle 
obtuse triangle  acute triangle 
The right triangle:  
The right triangle has one 90 degree angle and two acute (< 90 degree) angles. Since the sum of the angles of a triangle is always 180 degrees... y + z = 90 degrees The two sides of the triangle that are by the right angle are called the legs... and the side opposite of the right angle is called the hypotenuse. 

The equilateral triangle:  
In the equilateral triangle, all the sides are the same length (congruent) and all the angles are the same size (congruent). Since the sum of the angles of a triangle is always 180 degrees, we can figure out the measure of the angles of an equilateral triangle: 

The isosceles triangle:  
The isosceles triangle (I can NEVER remember how to spell isosceles) has two sides that are the same length (congruent) and two angles that are the same size (congruent). 
The obtuse triangle:  
Obtuse triangles aren't very smart. (Look up "obtuse" in the dictionary!)
Obtuse triangles have one angle that is greater than 90 degrees. (Obtuse triangles have one obtuse angle.) 

The acute triangle:  
Acute triangles are better looking than all the other triangles. (Pathetic attempt at a math joke.)
Acute triangles have NO angles greater than or equal to 90 degrees  all their angles are less than 90 degrees. (Acute triangles have all acute angles.) 