1 Complete the empty upper right part of two squares to get a large rectangle.

2 The area of a big rectangle minus three right triangles (1 is half the area of the big square, 2 is half the area of the small square, and 3 is the area of the right triangle completed above) is the area of the middle triangle.

Algorithm: (8+5) * 8-8 * 8 *1/2-5 *1/2-(8+5) * (8-5) *1/2 = 40.