*ANSWER* Q10 - into a Polar coordinate system

[arbipher]

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 "1-interval" demonstrated in the lecture.