Step 1 :

Surface integral .

Step 2 :

Evaluate .

Let the three vertices of the region be .

Then,

,

,

Thus, the plane is .

Rewrite the plane as

Step 3 :

.

Consider the domain as :

Consider

Differentiate the above equation partially with respect to x .

.

Consider .

Differentiate the above equation partially with respect to y .

.

Step 4 :

Now,

Thus, .