To figure this out, you just have to go back to the old formula of area = pi*r^2.  In this equation, r is the radius of the circle.

So, we know the area of the circle so we set up the equation

314 = pi*r^2.

So now we need to divide both sides by pi which is, happily enough, 3.14.  314/3.14 is 100.

So 100 equals r^2.  So to find r (the radius) we take the square root of 100, which is 10.

So the radius of the circle is 10 miles.