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Calculus / Chinese

Visualizing triple integrals

Charles队长 · Bilibili · 0:37

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This video demonstrates the calculation of a triple integral using the 'two-first-one-later' method (slicing). It visualizes the solid region bounded by a paraboloid and a plane, showing how horizontal cross-sections change with height. The specific integration limits and step-by-step evaluation are presented, yielding the final result.

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Chapters

0:00Problem Setup & Geometry0:08Slicing Method Visualization0:21Calculation Steps

Learning script

Generated from the video's visuals and explanation; not verbatim speech.

The video introduces a problem requiring the computation of a triple integral for the function (x2+y2)(x^2+y^2) over a solid region Ω\Omega. This region is defined as being enclosed below by the circular paraboloid z=x2+y2z=x^2+y^2 and above by the plane z=4z=4. A 3D coordinate system displays these surfaces.

It then explains the strategy of integrating with respect to xx and yy first (a double integral over a slice), followed by integrating with respect to zz. An animation shows a horizontal cutting plane moving up from the vertex at z=0z=0 to the top cap at z=4z=4, revealing that each cross-section is a disk whose radius depends on the current height zz.

Finally, the mathematical derivation is shown. The outer integral runs from z=0z=0 to z=4z=4. For the inner double integral over the disk D(z)D(z), polar coordinates are used where the angle goes from 00 to 2π2\pi and the radius rr goes from 00 to z\sqrt{z}. Substituting x2+y2=r2x^2+y^2=r^2 and the Jacobian rr, the integrand becomes r3r^3. Evaluating this gives 14z2\frac{1}{4}z^2, which leads to the final answer of 32π3\frac{32\pi}{3} after integrating along zz.

Knowledge cards

01

Region Definition

The domain of integration Ω\Omega lies between the surface z=x2+y2z = x^2 + y^2 and the flat lid z=4z = 4. Their intersection forms a circle of radius 2 located at height z=4z=4.

Ω={(x,y,z):x2+y2≤z≤4}\Omega=\{(x,y,z):x^2+y^2\le z\le4\}
02

Method of Slices

Also known as the 'first two, last one' approach. It computes the volume-weighted average by slicing the object horizontally. One integrates the function over the area of the slice A(z)A(z) first, then sums these slices along the vertical axis.

∭ΩfdV=∫cd(∬D(z)fdxdy)dz\iiint_{\Omega} f dV = \int_c^d \left( \iint_{D(z)} f dxdy \right) dz
03

Cross-Section Properties

At any fixed height zz, the intersection with the paraboloid creates a circular disk described by x2+y2≤zx^2 + y^2 \le z. In polar terms, this corresponds to radial bounds 0≤r≤z0 \le r \le \sqrt{z}.

D(z):x2+y2≤zD(z): x^2+y^2 \le z
04

Polar Integration

Converting the inner Cartesian double integral to polar coordinates simplifies the algebra significantly because both the boundary shape and the term x2+y2x^2+y^2 exhibit rotational symmetry around the z-axis.

x2+y2=r2,dxdy=rdrdθx^2+y^2=r^2, dxdy=rdrd\theta

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  • Definite integrals ApplicationAt 0:08
    Why this connection?

    The reviewed slicing card evaluates a triple integral by first integrating over the horizontal disk x2+y2≤zx^2+y^2\le z and then integrating in zz over 0≤z≤40\le z\le4. It applies iterated integration to the displayed paraboloid-and-plane region; polar coordinates require the area factor rr and radial bound 0≤r≤z0\le r\le\sqrt z.

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