数学试题讲解

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设

D = {(x, y) ∣ x^{2} + y^{2} \leq R^{2}}

, 则

\iint_{D} \sqrt{x^{2} + y^{2}} d σ =

π R^{3}

\frac{2 π R^{3}}{3}

π R^{2}

2 π R^{2}