Two congruent quadrilaterals and a rectangle. Which shaded region is larger, yellow or green?
As the line segment is common to both shapes, they are related by a rotation of about a point which can be found by joining two corresponding points, such as and , and finding its midpoint, which is . In particular, and are of equal length.
Since is parallel to , the perpendicular distances of and abvoe are equal, so triangles and have the same area. Then as and have the same length, triangles and have the same area. Putting these together shows that regions and have the same area. Therefore the yellow and green regions have equal area.