Answer:
A(r) = â2 * r
A(r) Domain is  R { r ;  r > 0}
Step-by-step explanation:
Diagonals of a square intercept each other in a 90° angle. The four triangles resulting from diagonal interception are equal and are isosceles triangles, with hipotenuse a side of the square
Therefore we apply  Pythagoras theorem
Let  x be side of square, and r radius of the circle, ( diagonals touch the circle) then
x²  =  r² + r²
x²  = 2r²
x  =  â2 * r
Now Aea of square is :
A  =  L²      where L is square side
A(r) Â = â2 * r
Domain of A(r) Â Â = Â R { r, r > 0}