College mathematics, the problem of triple integral, how did this circle become the next step?
It can be found that the integrand has nothing to do with X and Y, but only with Z, then the triple integral can be transformed into the double integral, that is, dxdy, and then the definite integral dz can be calculated. The integrand z 2 is equivalent to a constant in the double integral xoy plane, that is, the area of the integrand region, which is naturally the projection area of the ellipsoid in xoy, that is, that expression.