A cube contains 9 identical spheres, as shown below. There is one sphere in the center of the cube. Above (and below) the center sphere are 4 spheres tangent to the center sphere and the 4 corners of the cube.
What is the radius of each sphere?

A square contains a semicircle and a quarter circle, as shown. The two circles are tangent to each other. The semicircle has a diameter equal to 12 and the quarter circle has a radius equal to 12.
What is the area of the square?
What is the length of the tangent line segment AB?