If 1u = √(x2 + y2 + z2), then x∂u∂x + y∂u∂y + z∂u∂z is equal to:

If φ = f(y - z, z - x, x - y), then find the value of ∂φ∂x + ∂φ∂y + ∂φ∂z.

The point on the plane z = 2 + x + y that is closest to the point P(0, 1, 2) is:

If u = ln(x3 + y3x2 + y2), then find the value of x ∂u∂x + y ∂u∂y.