Second, angles on a straight line add up to 180 degrees. Using these two rules it's possible to prove that angles in a triangle add up to 180 Back to the bridge. To start, draw two parallel lines.

The sum of interior angles in a triangle is 180°. To prove \(a + b + c = 180^\circ\), firstly draw a line parallel to one side of the triangle. \(d = b\) (alternate angles are equal ...