2a∫[0,2a] x arccos(x/2a) dx
=2a( (x^2/2)arccos(x/2a) +∫(x^2/2) dx/√(4a^2-x^2) )[0,2a]
=a∫[0,2a] x^2/√(4a^2-x^2) dx
=a∫[0,π/2] 4a^2(sinθ)^2　dθ
=4a^3∫[0,π/2]∫(1-cos2θ)/2 dθ
=4a^3(θ/2 -sin2θ/4)[0,π/2]
=πa^3