Three mutually tangent spheres of radius rest on a horizontal plane. A sphere of radius rests on them. What is the distance from the plane to the top of the larger sphere?
Pick one
Solution
The height from the center of the bottom sphere to the plane is , and from the center of the top sphere to the tip is .
2004 AMC12A-22a.png
We now need the vertical height of the centers. If we connect centers, we get a rectangular pyramid with an equilateral triangle base. The distance from the vertex of the equilateral triangle to its centroid can be found by s to be .
2004 AMC12A-22b.png
By the Pythagorean Theorem, we have . Adding the heights up, we get
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.