Suppose that the area of a region D in the polar coordinate is A(D)=∫ 6
π
​
6
5π
​
​
∫ cscθ
2
​
rdrdθ. (1) (4 points) Find the average value of the function f(x,y)= (x 2
+y 2
) 3
​
1
​
on the region D.