The above double integral is evaluated on the square area bounded by and We divide this square region into sum of trapezoidal strips parallel to (See Figure 1)Putting and noting that the differential area between and is , the above double integral is converted to a single integral as follows:
and 
We divide this square region into sum of trapezoidal strips parallel to 
(See Figure 1)Putting 
and noting that the differential area between 
is 
, the above double integral is converted to a single integral as follows: