17. Four circles of radius r1 are inscribed inside a square of side 4r1 as shown in the diagram below. (a) What is the radius r2 of the largest circle that can fit in the space at the centre of the square, bounded by the outer circles? (b) If 8 spheres of radius r are now similarly arranged inside a cube of edge length 4r, what is the radius r3 of the largest sphere that can fit in the space at the centre of the cube? 12 4r 1 [3] [3]