A circle lies inside a quarter-circle, as shown below. The circle is tangent to side AO and arc AB. Find the radius of the circle.
Let the center of the small circle = M
Draw MN perpendicular to OA
We have right triangle ONM
MN^2 + ON^2 = OM^2
r^2 + 3^2 = (4 - r)^2
r^2 + 9 = r^2 - 8r + 16
8r = 7
r = 7/8
+33 
Find the radius of the circle.
