Question

Line segment $\overline{AB}$ is a diameter of a circle with $AB=24.$ Point $C,$ not equal to $A$ or $B,$ lies on the circle. As point $C$ moves around the circle, the centroid (center of mass) of $\triangle ABC$ traces out a closed curve missing two points. To the nearest positive integer, what is the area of the region bounded by this curve?

$25$

$38$

$50$

$63$

$75$

Answer: C

Concepts:centroid homothety circle area

Difficulty rating: 1600

Solution:

Let $O$ be the center of the circle. The centroid of $\triangle ABC$ is the average of $A,$ $B,$ and $C;$ since $O$ is the midpoint of $\overline{AB},$ the centroid lies one-third of the way from $O$ to $C.$

As $C$ traces the circle of radius $12,$ the centroid traces a circle of radius $\tfrac13\cdot12=4.$ Its area is $16\pi\approx50.$

Thus, the correct answer is C.

2018 AMC 12B Problem 8

Answer: C

Difficulty rating: 1600

Solution:

Problem 8 in Other Years