(x,y)与圆心距离为：d=√(x²+y²)
E(d)=1/(πR²)∫∫ √(x²+y²) dxdy
极坐标
=1/(πR²)∫∫ r² drdθ
=1/(πR²)∫[0→2π]dθ∫[0→R] r² dr
=(2/R²)(1/3)r³ |[0→R]
=(2/3R³)/R²
=2R/3
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