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The volume of the tetrahedron bounded by \( x + y + z = 1 \) and the coordinate planes is

  • A.\( 1 \)
  • B.\( 1/6 \)
  • C.\( 1/2 \)
  • D.\( 1/3 \)

Answer: B

Integrate the height \( z = 1 - x - y \) over the triangle \( x + y \le 1 \) in the base plane:

\[ V \;=\; \int_{0}^{1}\!\!\int_{0}^{1-x}\left(1 - x - y\right)dy\,dx \;=\; \int_{0}^{1}\frac{(1-x)^{2}}{2}\,dx \;=\; \frac{1}{6}. \]

The elementary formula \( \tfrac{1}{3}\times\text{base}\times\text{height} = \tfrac{1}{3}(\tfrac{1}{2})(1) \) agrees

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