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The average value of \( f(x,y) \) over a region \( R \) of area \( A \) is

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

Answer: A

As in one dimension, the average is the total divided by the extent over which it is spread:

\[ \bar{f} \;=\; \frac{1}{A}\iint_{R} f\,dA, \qquad A \;=\; \iint_{R} dA. \]

A constant integrand then returns its own value, which is the test any averaging formula must pass. The version with \( f^{2} \) leads to a root-mean-square instead

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