This part of the plane is a triangle. Call it
. We can find the intercepts by setting two variables to 0 simultaneously; we'd find, for instance, that
means
, so that (4, 0, 0) is one vertex of the triangle. Similarly, we'd find that (0, 5, 0) and (0, 0, 20) are the other two vertices.
Next, we can parameterize the surface by
so that the surface element is
Then the area of
is given by the surface integral