4. Let D C R' be an open set with regular, C' boundary, OD = c that is positively ori- ented (i.e. the unit normal obtained by counter-clockwise rotation of the unit tangent is inward pointing). (a) (2 points) Use Green's theorem to show that the area, A of D is A = xdy - ydx. C Hint: Determine P, Q such that O,Q -0, P = 1 and recall that A(D) = ff, 1dA. (b) (3 points) Calculate the area enclosed by a circle of radius r by calculating the line integral from the previous part. You must use the formula from the previous part. There are no points for calculating the area any other way