Observe that we have four right-angled triangles and a square packed into a large square. Each of the triangles has sides "a" and "b" and hypotenuse "c". The area of a square is defined as the square of the length of its sides - in this case, "(a + b)2". However, the area of the large square can also be expressed as the sum of the areas of its components. In this case, that would be the sum of the areas of the four triangles and the small square in the middle.