The square is inscribed in the circle.
So the diagonal of the square = diameter of circle = $2r$.
Let side of square be $a$.
Using diagonal formula:
$ a\sqrt{2} = 2r $
$ a = \dfrac{2r}{\sqrt{2}} = r\sqrt{2} $
Area of square:
$ a^2 = (r\sqrt{2})^2 = 2r^2 $
The shaded region is exactly half of the square (a triangular half).
So shaded area =
$ \dfrac{1}{2} \times 2r^2 = r^2 $
What is the diameter of the largest circle that can be drawn on a chessboard so that its entire circumference gets covered by the black squares and no part of the circumference falls on any white space, given that the chessboard has black and white squares of size one inch?