Robbins pentagon

The Robbins Pentagon is a shape made up of five sides (penta-means "five" and -gon just means "sides"). It was named after an American mathematician named Eliakim Hastings Robbins who discovered it in 1930. The Robbins Pentagon is special because it has an area that is equal to the total area of any triangles made outside of it. For example, if you draw three triangles outside of a Robbins Pentagon (no matter how big or small the triangles are), the total area of the three triangles will be the same as the area inside of the Robbins Pentagon.