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 degrees.

Isosceles Triangle

A triangle with two sides equal in size.
Isosceles Triangle
Angles B and C are equal in size.

Equilateral Triangle

A triangle with all sides equal in size.
Equilateral Triangle
All angles of the triangle are equal in size.

Right Triangle

A triangle with a right angle.
Right Triangle

c is the hypotenuse, a and b are the legs of the triangle.

Pythagorean theorem:

c 2 = a 2 + b 2

Sine law in a Triangle

Triangle without right angle may be solved using sine law expressed as follows
Right Triangle

a / sin(A) = b / sin(B) = c / sin(C)

Cosine law in a Triangle

cosine law 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 geometry references
Triangle Problems

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Updated: 2 April 2013

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