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.

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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:

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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).

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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.)

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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.)