Two adjacent sides of a square and an arc are regarded as 1/4 of a circle whose radius is the side length of the square.

The area of this 1/4 garden = 10? *π/4=25π

Half of the shadow area in the figure can be regarded as 1/4 circle area-square area /2 (triangle area with square side).

=25π- 10* 10/2=25π-50

So the shadow area in the figure = 2 * (25 π-50) = 50 π-100 = 50 * 3.1416-100 = 57.08.