see the attached figure to better understand the problem
we know that
the area of a square is equal to

where
b is the length side of the square
in this problem, the length side of the square is equal to the diameter of the circle
so
b=2r
<u>substitute in the formula of the area of the square</u>


<u>Find the area of the circle</u>
the area of the circle is equal to

<u>Find the area of the grass</u>
the area of the grass is the area of the square minus the area of the circle
so
![A=4r^{2} - \pi r^{2} = r^{2}*[4-\pi ]](https://tex.z-dn.net/?f=%20A%3D4r%5E%7B2%7D%20-%20%5Cpi%20r%5E%7B2%7D%20%3D%20r%5E%7B2%7D%2A%5B4-%5Cpi%20%5D)
therefore
<u>the answer is</u>
the area A of the grass as a function of r is equal to
![A=r^{2}*[4-\pi ]\ units^{2}](https://tex.z-dn.net/?f=%20A%3Dr%5E%7B2%7D%2A%5B4-%5Cpi%20%5D%5C%20units%5E%7B2%7D%20)