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Ch. 6 - Applications of Integration
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Not the one you use?Change textbook
All textbooksBriggs 3rd EditionCh. 6 - Applications of IntegrationProblem 6.3.31
Chapter 6, Problem 6.3.31

Let R be the region bounded by the following curves. Find the volume of the solid generated when R is revolved about the given axis.
y=sin xon [0,π] and y=0 ; about the x-axis (Hint: Recall that sin^2 x=1 − cos2x / 2.

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Identify the region R bounded by the curves y = sin x and y = 0 on the interval [0, \(\pi\)]. This region lies between the curve y = sin x and the x-axis from x = 0 to x = \(\pi\).
Since the region is revolved about the x-axis, use the disk method to find the volume. The volume V is given by the integral formula: \(V = \pi \int_0^{\pi} [f(x)]^2 \, dx\) where \(f(x) = \sin x\) is the radius of the disk at position x.
Substitute \(f(x) = \sin x\) into the volume integral: \(V = \pi \int_0^{\pi} (\sin x)^2 \, dx\).
Use the given trigonometric identity to simplify the integrand: \(\sin^2 x = \frac{1 - \cos 2x}{2}\) Rewrite the integral as: \(V = \pi \int_0^{\pi} \frac{1 - \cos 2x}{2} \, dx\).
Split the integral into two simpler integrals and set up the expression for evaluation: \(V = \frac{\pi}{2} \int_0^{\pi} (1 - \cos 2x) \, dx = \frac{\pi}{2} \left[ \int_0^{\pi} 1 \, dx - \int_0^{\pi} \cos 2x \, dx \right]\).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Volume of Solids of Revolution

This concept involves finding the volume of a 3D solid formed by rotating a 2D region around an axis. The disk or washer method is commonly used, where cross-sectional areas perpendicular to the axis of rotation are integrated over the interval. For rotation about the x-axis, the volume is found by integrating π times the square of the function representing the radius.
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Finding Volume Using Disks

Trigonometric Identities

Trigonometric identities simplify expressions involving trigonometric functions. In this problem, the identity sin²x = (1 - cos 2x)/2 helps transform the integrand into a more manageable form for integration. Using such identities is essential to evaluate integrals involving powers of sine or cosine.
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Verifying Trig Equations as Identities

Definite Integration over an Interval

Definite integration calculates the exact area under a curve between two points, here from 0 to π. It is used to sum the infinitesimal volumes of disks or washers to find the total volume. Understanding how to set up and evaluate definite integrals is crucial for solving volume problems in calculus.
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Related Practice
Textbook Question

45–48. Shell and washer methods about other lines Use both the shell method and the washer method to find the volume of the solid that is generated when the region in the first quadrant bounded by y = x²,y=1, and x=0 is revolved about the following lines. 


x = -1

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Textbook Question

Let R be the region bounded by the following curves. Find the volume of the solid generated when R is revolved about the given axis.

y=x,y=x+2,x=0, and x=4 ; about the x-axis

Textbook Question

Find the area of the region described in the following exercises.


The region bounded by y=4x+4, y=6x+6, and x=4

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Textbook Question

Find the arc length of the line y = 4−3x on [−3, 2] using calculus and verify your answer using geometry.

Textbook Question

Let R be the region bounded by the following curves. Find the volume of the solid generated when R is revolved about the given line.


y=x and y=1+x/2; about y=3

Textbook Question

Look again at the region R in Figure 6.38 (p. 439). Explain why it would be difficult to use the washer method to find the volume of the solid of revolution that results when R is revolved about the y-axis.

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