**A triangle is a three sided
polygon. A polygon is a closed figure in a plane that is
made up of segments called sides that intersect only at their endpoints,
called vertices. **

**Parts of a Triangle**

**One way to classify triangles
is by their angles. **

**All triangles have at least
two acute angles, but the third can be acute, right, or obtuse.
A triangle can be classified by using the third angle.**

__Acute triangle__- all the angles of the triangle are
acute. Click here for intermath description for acute
triangle.

__Obtuse triangle__- one angle of the triangle is obtuse.

__Right triangle__- one angle of the triangle is right.

__Equiangular triangle__- a triangle in which all the angles
of the triangle are equal. Click here for intermath info on equilateral
triangle.

**Triangles can also be classified
by according to the number of congruent sides.**

__Scalene triangle__- a triangle in which no two sides
are congruent.

__Isosceles triangle__- a triangle in which at least two
sides are congruent.

__Equilateral triangle__- a triangle in which all the sides
are congruent.

**Click here for intermath description for scalene,
isosceles, and equilateral triangles.**

**Some parts of a right triangle and
isosceles triangle have special names.**

**Click ****here
for right triangle parts.**

**Click ****here
for isosceles triangle parts.**