( 843, #35) The average value of a function f(x,y) over a rectangle
R is defined to be
where A(R) is the area of R. (Compare with the definition
for functions of one variable in Equation 5.5.1.) Find the average value
of f over the rectangle 
.
Solution: This is the graph of the function bounded by the planes of the edges of the rectangle R.
The area of R is the length times the width:
[ 1 - (-1)]*[5 - 0] = 2*5 = 10
The average value of the function is:
by: nl