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Geometry Difficulty 4.9 AIME Prove it Brazil

Show that the maximum number of spheres of radius 11 that can be placed touching a fixed sphere of radius 11 so that no pair of spheres has an interior point in common is between 1212 and 1414.

Solution

We can place 66 spheres with their centers coplanar with the fixed sphere. Then we can place 33 more above and 33 more below as shown above. Thus 1212 can be achieved.

Figure 1

Take OO to be the center of the fixed sphere. Another sphere touching it blocks off a conical solid angle as shown. The angle between the center line of the cone and the surface is 3030^\circ. It's not hard to show that for an angle θ\theta the solid angle is 2π(1cosθ)2\pi(1 - \cos\theta), so for θ=30\theta = 30^\circ, it is π(23)\pi(2 - \sqrt{3}). Thus we can have at most 4ππ(23)=4(2+3)<15\frac{4\pi}{\pi(2-\sqrt{3})} = 4(2+\sqrt{3}) < 15 spheres touching the fixed sphere. Hence at most 1414.

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