>>539-540

  xy -(x+y+1) +2 = (x-1)(y-1) ≦ 2,
∴ xyz = xy(11-x-y) ≦ (1+x+y)(11-x-y) = 36 - (5-x-y)^2 ≦ 36,
∴ √(xyz) ≦ 6,