How can we prove that the angles in at triangle equal the same total as the angles on a straight line? We used a variety of triangles to prove it. Somebody commented: ‘I know that the angles in a square add up to 360 degrees. A triangle is half of a square, so the total of the angles in a triangle must be half of the total angles in a square.’ This is amazing reasoning! How could we prove this with a diagram of a square?