Triangles (Mathematics Glossary)
What Is a Triangle?A triangle is a 2-dimensional shape with three sides and three angles.
Dictionary DefinitionThe Oxford English Dictionary defines a triangle as "a figure (usually, a plane rectilineal figure) having three angles and three sides."
Types of TriangleThere are different types of triangle.
Triangles can be classified by lengths of sides or by angles.
By Lengths of Sides:
Triangles can be classified as equilateral, isosceles or scalene depending on whether 3, 2 or 0 side lengths and angles are equal.
Triangles can be classified as acute, obtuse or right triangles depending on whether all angles are acute (less than 90°), one angle is obtuse (more than 90°) or one angle is a right angle (90°).
Read more about types of triangle
Properties of Triangles
Area of a TriangleThe area of a triangle is found using the formula:
In this formula, b is the length of the base of the triangle and h is the height of the triangle.
Read more about how to find the area of a triangle
Interior Angles of a TriangleThe interior angles of a triangle add up to 180°.
Read more about the interior angles of a triangle
Here's a second test on the triangle.
Here's a third test on the triangle.
What's in a Name?A triangle has three corners and so three angles. Just as a tricycle has three wheels, and a tripod has three legs, a shape with three angles in called a triangle.
An alternative name for a triangle is a 'trigon', as it is a three-sided polygon. This is where we get the word trigonometry.
Oblique TrianglesA triangle without a right angle is called an oblique triangle.
Acute triangles and obtuse triangles are both oblique triangles.
Finding the Area of a Triangle Using TrigonometryThe standard way to find the area of a triangle is:
½ × base × heightAn alternative way to calculate the area of a triangle is using trigonometry.
The area of a triangle is given by the formula:
In this formula, a and b are two sides of the triangle, and C is the angle between these sides.
Read more about how to find the area of a triangle using trigonometry