A circle with a diameter of 10 has a radius of 5. The area of a circle is the radius squared times . 5 squared is 25, and since is more than 3, the area has to be more than 3 times 25 = 75 and since is less than 4, the area has to be smaller than 4 times 25 = 100. But a square has an area that is the measure of the side squared. 10 squared is 100. So which has a larger area?
Another way to look at it is this. If a square measures 10 on a side, then it also measures 10 across the center. So if you take a circle with a diameter of 10 and put it on top of the square so their centers coincide, the circle would just fit. But the circle wouldn't cover the entire square - you would have those 4 sort-of triangle shaped pieces in the corners that weren't covered.
