Sun K.

asked • 03/24/13# Use the Divergence Theorem to compute the surface integral?

Use the Divergence Theorem to compute the surface integral where Q is bounded by z=x^2+y^2 and z=4, F=<x^3, y^3-z, xy^2>. (Answer: 32pi)

The divergence of the vector field is 3x^2+3y^2, which I've found.

How should I set up the triple integral and find the points of the integral and solve for it?

And where did you get 2*pi*r from?

## 1 Expert Answer

Robert J. answered • 03/24/13

Certified High School AP Calculus and Physics Teacher

div **F** = ∂(x^3)/∂x + ∂(y^3-z)/∂y + ∂(xy^2)/∂z = 3x^2 + 3y^2

By the Divergence Theorem ∫∫**F⋅N** dS = ∫∫∫div **F** dV,

the surface integral

= ∫∫∫3(x^2+y^2) dx dy dz

= ∫[0,4]∫[0, sqrt(z)] 3(r^2) 2pi*r dr dz

= ∫[0,4] (3/2) pi z^2 dz

= (1/2) pi z^3 from 0 to 4

= 32 pi <==Answer

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Sun K.

How did you get from 0 to sqrt(z)?

03/24/13