Find the average value of f(x,y) = sin (x + y) over a. the rectangle 0 x/3, 0 5/3 b. the rectangle 0 x2/3, 0s7/6 a. The average value of f is 0 0. (Type an exact answer, using it as needed.) b. The average value of f is . (Type an exact answer, using it as needed.)