Since the length of the square is l, the area would be l*l = l².
The area of the circle is given by the formula A=πr². In this case our radius, r, is 1/2 the length of the square, or 1/2l. This makes the area of the circle A=π(1/2l)²=π(1/4l²)=π(1/4)(l²)=1/4πl². (Remember, multiplication is commutative, which means we can switch these around to make them easier for us.)
The wasted area would be the area of the square minus the used area of the circle, or
l²-1/4πl²