Question

A regular octahedron has side length $1.$ A plane parallel to two of its opposite faces cuts the octahedron into two congruent solids. The polygon formed by the intersection of the plane and the octahedron has area $\dfrac{a\sqrt{b}}{c},$ where $a,$ $b,$ and $c$ are positive integers, $a$ and $c$ are relatively prime, and $b$ is not divisible by the square of any prime. What is $a + b + c?$

$10$

$11$

$12$

$13$

$14$

Answer: E

Concepts:3D geometry regular polygon area

Difficulty rating: 2270

Solution:

Let the two parallel faces be triangles. The plane passes through the midpoints of the six edges not on those faces, forming an equilateral hexagon of side $\dfrac{1}{2},$ which by symmetry is also equiangular and hence regular.

A regular hexagon is six equilateral triangles, so its area is $6\cdot\frac{\sqrt{3}}{4}\left(\frac{1}{2}\right)^2 = \frac{3\sqrt{3}}{8}.$

Thus $a = 3,$ $b = 3,$ $c = 8,$ and $a + b + c = 14.$

Thus, the correct answer is E.

2009 AMC 12A Problem 22

Answer: E

Difficulty rating: 2270

Solution:

Problem 22 in Other Years