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Question Define \( Q \) as the region bounded on the right by the graph of \( f(y)=\frac{3}{y} \) and on the left by the \( y \)-axis for \( y \in[1,3] \). Find the volume of the solid of revolution formed by revolving \( Q \) around the \( x \)-axis. - Enter an exact value in terms of \( \pi \).

Ask by Collins Kelly. in the United States
Jan 23,2025

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Answer

The volume of the solid of revolution is \( 6\pi \) cubic units.

Solution

To find the volume of the solid of revolution formed by revolving the region \( Q \) around the \( x \)-axis, we can use the method of disks. The volume of the solid of revolution is given by the formula: \[ V = \pi \int_{a}^{b} [f(y)]^2 \, dy \] In this case, the function \( f(y) = \frac{3}{y} \) and the region \( Q \) is bounded on the right by the graph of \( f(y) \) and on the left by the \( y \)-axis for \( y \in [1,3] \). Substitute \( f(y) = \frac{3}{y} \) into the formula: \[ V = \pi \int_{1}^{3} \left(\frac{3}{y}\right)^2 \, dy \] Now, we can calculate the integral to find the volume of the solid of revolution. Evaluate the integral by following steps: - step0: Evaluate using formulas and rules: \(\pi \times \int_{1}^{3} \left(\frac{3}{y}\right)^{2} dy\) - step1: Evaluate the integral: \(\pi \times 6\) - step2: Reorder the terms: \(6\pi \) The volume of the solid of revolution formed by revolving the region \( Q \) around the \( x \)-axis is \( 6\pi \) cubic units.

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The Deep Dive

To find the volume of the solid of revolution formed by revolving the region \( Q \) around the \( x \)-axis, we will use the disk method. The volume \( V \) is given by the integral: \[ V = \pi \int_{y_1}^{y_2} [f(y)]^2 \, dy \] In this case, \( f(y) = \frac{3}{y} \), and the boundaries are \( y_1 = 1 \) and \( y_2 = 3 \). Thus, we set up the integral as follows: \[ V = \pi \int_{1}^{3} \left( \frac{3}{y} \right)^2 \, dy \] Calculating \( \left( \frac{3}{y} \right)^2 \): \[ \left( \frac{3}{y} \right)^2 = \frac{9}{y^2} \] Now, we can write the volume integral: \[ V = \pi \int_{1}^{3} \frac{9}{y^2} \, dy \] Next, we calculate the integral: \[ V = \pi \cdot 9 \int_{1}^{3} y^{-2} \, dy \] The integral of \( y^{-2} \) is: \[ \int y^{-2} \, dy = -\frac{1}{y} \] Evaluating the definite integral from \( 1 \) to \( 3 \): \[ = -\frac{1}{3} - (-\frac{1}{1}) = -\frac{1}{3} + 1 = \frac{2}{3} \] Now substituting back into the volume formula: \[ V = \pi \cdot 9 \cdot \frac{2}{3} = \pi \cdot 6 = 6\pi \] Hence, the volume of the solid of revolution is: \[ \boxed{6\pi} \]

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