Answer
$A_{ring} = \pi BC^2$
Work Step by Step
We call r the radius of the smaller circle. The radius of the larger circle is:
$ R = \sqrt{BC^2 + r^2}$
Thus, the area of the ring is:
$A = \pi (R^2 - r^2) \\ A = \pi (BC^2 + r^2 - r^2) = \pi BC^2$
