Question

A semicircle of diameter $1$ sits at the top of a semicircle of diameter $2,$ as shown. The shaded area inside the smaller semicircle and outside the larger semicircle is called a lune. Determine the area of this lune.

$\dfrac{1}{6}\pi - \dfrac{\sqrt{3}}{4}$

$\dfrac{\sqrt{3}}{4} - \dfrac{1}{12}\pi$

$\dfrac{\sqrt{3}}{4} - \dfrac{1}{24}\pi$

$\dfrac{\sqrt{3}}{4} + \dfrac{1}{24}\pi$

$\dfrac{\sqrt{3}}{4} + \dfrac{1}{12}\pi$

Answer: C

Concepts:sector area decomposition equilateral triangle

Difficulty rating: 1660

Solution:

The small semicircle's diameter is a chord of length $1$ in the large circle. Joining its endpoints to the large circle's center gives an equilateral triangle of side $1$ and area $\dfrac{\sqrt{3}}{4}.$

The region between the chord and the small arc, taken together with that triangle, has area $\dfrac{\sqrt{3}}{4} + \dfrac{1}{2}\pi\left(\dfrac{1}{2}\right)^2 = \dfrac{\sqrt{3}}{4} + \dfrac{\pi}{8}.$

Subtracting the $60^\circ$ sector of the large circle, of area $\dfrac{1}{6}\pi(1)^2 = \dfrac{\pi}{6},$ leaves the lune: $\dfrac{\sqrt{3}}{4} + \dfrac{\pi}{8} - \dfrac{\pi}{6} = \dfrac{\sqrt{3}}{4} - \dfrac{\pi}{24}.$

Thus, the correct answer is C.

2003 AMC 10A Problem 19

Answer: C

Difficulty rating: 1660

Solution:

Problem 19 in Other Years