Math Problem Statement

Q8. Evaluate ∭E z dV. E is the solid bounded by z=2-x^2-y^2 and z=1.

Solution

To evaluate the triple integral EzdV\iiint_E z \, dV, where EE is the solid bounded by z=2x2y2z = 2 - x^2 - y^2 and z=1z = 1, we will use cylindrical coordinates for simplicity.

In cylindrical coordinates (r,θ,z)(r, \theta, z), the boundaries transform as follows:

  • x=rcosθx = r \cos \theta
  • y=rsinθy = r \sin \theta
  • z=zz = z
  • The Jacobian determinant for the transformation to cylindrical coordinates is rr.

The solid EE is bounded by z=1z = 1 (bottom surface) and z=2r2z = 2 - r^2 (top surface).

  1. The bounds for zz are from 11 to 2r22 - r^2.
  2. The bounds for rr are determined by the projection of the region onto the xyxy-plane. At z=1z = 1, 1=2r21 = 2 - r^2, so r=1=1r = \sqrt{1} = 1. Thus, rr ranges from 00 to 11.
  3. The bounds for θ\theta are from 00 to 2π2\pi.

The integral in cylindrical coordinates becomes: EzdV=02π0112r2zrdzdrdθ\iiint_E z \, dV = \int_0^{2\pi} \int_0^1 \int_1^{2-r^2} z \, r \, dz \, dr \, d\theta

First, integrate with respect to zz:

= \frac{(4 - 4r^2 + r^4)}{2} - \frac{1}{2} = 2 - 2r^2 + \frac{r^4}{2} - \frac{1}{2} = \frac{3}{2} - 2r^2 + \frac{r^4}{2}$$ Now, the integral becomes: $$\int_0^{2\pi} \int_0^1 \left( \frac{3}{2} - 2r^2 + \frac{r^4}{2} \right) r \, dr \, d\theta$$ Separate the integral: $$\int_0^{2\pi} d\theta \int_0^1 \left( \frac{3}{2}r - 2r^3 + \frac{r^5}{2} \right) dr$$ Evaluate the $$\theta$$ integral: $$\int_0^{2\pi} d\theta = 2\pi$$ Now, evaluate the $$r$$ integral: \[ \int_0^1 \left( \frac{3}{2}r - 2r^3 + \frac{r^5

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Math Problem Analysis

Mathematical Concepts

Triple Integrals
Cylindrical Coordinates

Formulas

Triple integral in cylindrical coordinates

Theorems

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Suitable Grade Level

Advanced Undergraduate