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Revolving \( y = x \) from \( x = 0 \) to \( x = 3 \) about the \( x \)-axis gives a volume of

  • A.\( 9\pi/2 \)
  • B.\( 9\pi \)
  • C.\( 3\pi \)
  • D.\( 27\pi \)

Answer: B

Apply the disc formula with \( y = x \):

\[ V \;=\; \pi\int_{0}^{3} x^{2}\,dx \;=\; \pi\left[\frac{x^{3}}{3}\right]_{0}^{3} \;=\; 9\pi. \]

The solid is a cone of radius three and height three, and the elementary formula \( \tfrac{1}{3}\pi r^{2}h = \tfrac{1}{3}\pi(9)(3) = 9\pi \) agrees

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