Math Gold Medalist

Lor

2023 AMC 10A

Problem 15

An even number of circles are nested, starting with a radius of $1$ and increasing by $1$ each time, all sharing a common point. The region between every other circle is shaded, starting with the region inside the circle of radius $2$ but outside the circle of radius $1.$ An example showing $8$ circles is displayed below. What is the least number of circles needed to make the total shaded area at least $2023\pi$?

[asy] size(6cm); pen greywhat; greywhat = RGB(105,105,105); filldraw(circle((8, 0), 8), greywhat); filldraw(circle((7, 0), 7), white); filldraw(circle((6, 0), 6), greywhat); filldraw(circle((5, 0), 5), white); filldraw(circle((4, 0), 4), greywhat); filldraw(circle((3, 0), 3), white); filldraw(circle((2, 0), 2), greywhat); filldraw(circle((1, 0), 1), white); [/asy]

$\textbf{(A) } 46 \qquad \textbf{(B) } 48 \qquad \textbf{(C) } 56 \qquad \textbf{(D) } 60 \qquad \textbf{(E) } 64$


Area

Important Identities

Arithmetic Sequence

.

If there are 2k circles then

Area = pi(2^2-1^2)+pi(4^2-3^3)+…+pi((2k)^2-(2k-1)^2)

(2k+1)k>=2023

   Solution