>>776
A = O,
E(p,q,0):　OE↑ = OC↑ - {zc/(zc-zb)}BC↑,
F(r,s,0):　OF↑ = OD↑ - {zd/(zd-zb)}BD↑,
△AEF = |ps-qr|/2,
高さ h = |zb|,
体積 = (1/3)h･△AEF,