Maths Olympiad Prep

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Geometry Difficulty 3.8 AMC 10/12 Find the answer

Three mutually tangent spheres of radius 11 rest on a horizontal plane. A sphere of radius 22 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 11, and from the center of the top sphere to the tip is 22.
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 30-60-9030\text-60\text-90 \triangles to be 233\frac{2\sqrt{3}}{3}.
2004 AMC12A-22b.png
By the Pythagorean Theorem, we have (23)2+h2=32h=693\left(\frac{2}{\sqrt{3}}\right)^2 + h^2 = 3^2 \Longrightarrow h = \frac{\sqrt{69}}{3}. Adding the heights up, we get 693+1+2(B) 3+693\frac{\sqrt{69}}{3} + 1 + 2 \Rightarrow\boxed{\mathrm{(B)}\ 3+\frac{\sqrt{69}}{3}}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.