See attached graphic.
Triangle ABC is a 45 45 90 triangle.
Side AB = Side BC = hypotenuse * (1 / sq root (2) )
SIde AB = BC = 10 *
0.7071067812
SIde AB = BC = 7.071067812
Area of Triangle ABC = (AB * BC) / 2
Area of Triangle ABC = 25
Area of Triangle DEF = 25
Line FB = 10
Line FD = 7.071067812
Area of Rectangle FBDA = 70.71067812
Total Area of Shaded Area = 25 + 25 + 70.71067812 =
120.71067812
