This seems trickier than it really is
Look at the following figure :
![](/img/upload/cdd79523607369d8f51f3e3c/capture-1.png)
Note that since the sides of the square = 18....then DB = sqrt 2*18 = AC
E is the center of the square so BE = CE = sqrt (2)*10
And angle CEB = 90°
And the circle has a radius of 18
So....if we take the area of 1/2 of this circle and subtract the area of triangle CEB we will have the same area as the shaded region
Area of half-circle with radius of 12 = pi (6/2)^2 / 2 = 18*pi
Area of triangle CEB = area of an isosceles triangle with equal sides of sqrt (2)/2 and an included angle of 90° =
(1/2) [ sqrt (2) / 2 ] ^2 *sin 90° = 12
So....the shaded area = 16*pi - 4