取x轴上半个圆环，显然质心横坐标=0换为极坐标求纵坐标，取密度ρ=1，用1/4圆环计算y0=∫(0,π/2)dθ∫(1,2)ρyrdr / ∫(0,π/2)dθ∫(1,2)ρrdr=∫(0,π/2)sinθdθ∫(1,2)r^2dr / ∫(0,π/2)dθ∫(1,2)rdr=7/3/(3π/4)=28/9π
