Angles on a straight line also add up to 180°. If a straight line has been split into three separate angles, it looks as if three corners of a triangle have been put together - just like in this ...

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 ...