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The volume under the surface \( z = f(x,y) \) above a region \( R \) is

  • A.\( \pi\displaystyle\iint_{R} f^{2}\,dA \)
  • B.\( \displaystyle\iint_{R} \sqrt{1 + f^{2}}\,dA \)
  • C.\( \displaystyle\iint_{R} f\,dA \)
  • D.\( \displaystyle\iint_{R} f^{2}\,dA \)

Answer: C

Each element of the base carries a column of height \( f \) and base area \( dA \), so its volume is \( f\,dA \) and

\[ V \;=\; \iint_{R} f(x,y)\,dA. \]

The squared integrand with \( \pi \) belongs to a solid of revolution, whose slices are discs rather than columns, and the square root form gives a surface area

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