# Triangles

Definitions and properties of triangles in geometry.

## Angles of a Triangle

A triangle is a closed plane figure with three (straight) sides. The sum of all three interior angles of a triangle is equal to 180�.

## Isosceles Triangle

An isosceles triangle is a triangle with two equal sizes. In the above triangle sides AC and BC are equal and therefore angles A and B are also equal.

## Equilateral Triangle

An equilateral triangle has all sides and all angles are equal in size. All angles of the triangle are equal to 60�.

## Right Triangle

A right triangle has a right angle. c is the hypotenuse, a and b are the sides of the triangle.

### Pythagorean Theorem

In any right triangle with hypotenuse c and sides a and b we have:
c 2 = a 2 + b 2

## Sine law in a Triangle

Triangle without right angle may be solved using sine law expressed as follows a / sin(A) = b / sin(B) = c / sin(C)

## Cosine law in a Triangle

The cosine is alos used to solve traingles without right angle. The cosine law is expressed as follows:
a 2 = b 2 + c 2 - 2 b c cos(A)
or b 2 = a 2 + c 2 - 2 a c cos(B)
or c 2 = b 2 + a 2 - 2 b a cos(C)

## More References and Links to Triangles

Triangle Problems
Geometry Tutorials, Problems and Interactive Applets.