What haveyoutried?

I will give you this much: we can write the cone in parametric coordinates using polar coordinates: , , with from 0 to H and [/tex]\theta[/tex] from 0 to .

With that you can write the "position vector" of a point on the surface of the cone as .

The deriviatives of that with respect to r and :

and

are in the tangent plane to the cone and their cross product is

.

The (upward oriented) "vector differential of area" is

Take the dot product of your field with that and integrate.

And whatis"Gauss's theorem"?