Triangle Information
Topic Index | Geometry Index | Regents Exam Prep Center


Triangles can be classified in two ways:
by Sides and by Angles.


Classified by Sides:

A Scalene Triangle has no congruent sides.
Each side in this triangle has a different length.


An Isosceles Triangle has two congruent sides.

The sides that are the same length are called the legs.
The other side is called the base.

An Equilateral Triangle has three congruent sides.

All three sides of this triangle are of the same length.


Classified by Angles:


An Acute Triangle
has all angles measuring less than 90°.



A Right Triangle has one right angle.
(A right angle measures exactly 90°).


An Obtuse
Triangle has one angle measuring more than 90°.
It is not possible to draw a triangle with more than one obtuse angle.



An Equiangular
Triangle has 3 congruent angles.
All angles measure the same, and each angle measures 60°.
This triangle is the same as the equilateral triangle.


Area of Triangles:

Finding the area of a triangle can be as simple as plugging numbers into the well known formula for the area of a triangle:

But how do you find the area of a triangle when you do not know the height of the triangle?

A Greek engineer and geometer, Hero (or Heron) of Alexandria, who lived in the first century 10 - 75, is credited with discovering a means of finding the area of a triangle when the height is not known.

Heron's formula: 

where a, b and c are the sides of the triangle, and

s is called the semi-perimeter.