Let S be the part of the sphere

x^2 + y^2 + z^2 = 25
that lies above the plane z = 4. Let S have constant density k.
(a) Find the center of mass.

(b) Find the moment of inertia about the z-axis.

To find the center of mass and the moment of inertia about the z-axis for the given surface, we need to use the formulas related to triple integrals.

(a) Center of Mass:
The center of mass of a solid with constant density is given by the following formula:

(x̄, ȳ, ẑ) = (1/M) ∫∫∫ (x, y, z) ρ(x, y, z) dV

where (x̄, ȳ, ẑ) represents the center of mass coordinates, M is the total mass, (x, y, z) are the coordinates of a point inside the solid, ρ(x, y, z) is the density function, and dV represents the differential volume element.

Given that the density function is constant (k) and the solid S is part of a sphere, the density function becomes ρ(x, y, z) = k. Also, the equation of the sphere is given as x^2 + y^2 + z^2 = 25.

To find the center of mass, we integrate over the region S that lies above the plane z = 4:

(x̄, ȳ, ẑ) = (1/M) ∫∫∫ (x, y, z) k dV

We can convert this triple integral into spherical coordinates to simplify the calculations. The limits of integration for the spherical coordinates are:

0 ≤ ρ ≤ 5, 0 ≤ θ ≤ 2π, arccos(4/5) ≤ φ ≤ π/2

The differential volume element in spherical coordinates is given by dV = ρ^2 sin(φ) dρ dθ dφ.

Now we can set up the integral to calculate the center of mass:

(x̄, ȳ, ẑ) = (1/M) ∫∫∫ (x, y, z) k ρ^2 sin(φ) dρ dθ dφ

where the integrals are evaluated over the limits mentioned above.

(b) Moment of Inertia about the z-axis:
The moment of inertia about the z-axis for a solid with constant density is given by the following formula:

Iz = ∫∫∫ (x^2 + y^2) ρ(x, y, z) dV

For our given surface, using spherical coordinates again, the moment of inertia can be calculated as:

Iz = ∫∫∫ (ρ^2 sin^2(φ) (sin^2(θ) + cos^2(θ))) k ρ^2 sin(φ) dρ dθ dφ

where the integrals are evaluated over the same limits as before.