Question

A $4\times4\times h$ rectangular box contains a sphere of radius $2$ and eight smaller spheres of radius $1.$ The smaller spheres are each tangent to three sides of the box, and the larger sphere is tangent to each of the smaller spheres. What is $h?$

$2+2\sqrt7$

$3+2\sqrt5$

$4+2\sqrt7$

$4\sqrt5$

$4\sqrt7$

Answer: A

Concepts:3D geometry sphere Pythagorean Theorem

Difficulty rating: 1800

Solution:

Place the box with a corner at the origin. Each small sphere sits in a corner with center $1$ unit from three faces. The four top small-sphere centers form a square of side $2,$ whose center lies on the box axis; a corner of that square is $\sqrt2$ from the center.

The big sphere's center is on the axis, at distance $2+1=3$ from each top small center. The vertical gap between them is $\sqrt{3^2-\sqrt2^2}=\sqrt7.$

The big center is at height $\dfrac h2$ and the top small centers at height $h-1,$ so $(h-1)-\dfrac h2=\sqrt7,$ giving $\dfrac h2=1+\sqrt7$ and $h=2+2\sqrt7.$

Thus, the correct answer is A.

2014 AMC 12A Problem 17

Answer: A

Difficulty rating: 1800

Solution:

Problem 17 in Other Years