*ANSWER* Q10  into a Polar coordinate system
I find it's easy to solve the question in a polar coordinate system,
in which we can depict an arbitrary point as:
(r, theta)
, where r is the radius and theta is the angle.
The region in two concentric circles is
(r, theta)
, where a <= r <= b and theta is arbitrary.
Now it's easy to realize that the answer to this question should be identical to "1interval" demonstrated in the lecture.
