The part that is cut off is asking the area of the shaded section of the circle. How do I solve this problem only knowing one side of the square?
The radius of that circle is 1. The angle at O is $\displaystyle \tfrac{\pi}{2}$.
Thus the shaded area is $\displaystyle \tfrac{1}{4}$ the area of the circle.
I understand that the radius is 1, due to the side of the square being the diameter. However, I don't understand how you found O's angle. Can you explain, please?