Math is Fun
Math is Fun Home PageHOME

A-Z Listing



Top-rated teacher Web sites rated by teacher usage
Edunet site of the week January 2001
Maths Is Fun is ICRA Registered

Copyright © 2007
MathsIsFun.com

Triangles Contain 180°

This is a proof that the angles in a triangle equal 180°:

Proof the angles in a triangle add to 180

Explanation 1: The top line (that touches the top of the triangle) is running parallel to the base of the triangle. So:

  • angles "a" are the same, and
  • angles "b" are the same,

and you can easily see that "a" + "b" + "c" does a complete rotation from one side of the straight line to the other, or 180°



Explanation 2: From the rules of angles, when a line cuts two parallel lines we can show that the angles of the triangle a + b + c = the angle on a straight line = 180°


Tell A Friend Add to Favorites Link to Us