{ diff(x1, t) = (1*k20*k2*k3^k4/(k3^k4 + x3^k4) + (-1)*k20*k5*x1/(k6 + x1) + 1*k20*k7*k8*1/2*(x4 + x8)/(k9 + k8*1/2*(x4 + x8)) + 1*k20*k10)/k20,
  diff(x2, t) = (1*k20*k11*x1 + (-1)*k20*k12*x2/(k13 + x2))/k20,
  diff(x3, t) = (1*k20*k14*x2 + (-1)*k20*k15*x3/(k16 + x3))/k20,
  diff(x4, t) = (1*k20*k17*x1 + (-1)*k20*k18*x4/(k19 + x4))/k20,
  diff(x5, t) = (1*k20*k2*k3^k4/(k3^k4 + x7^k4) + (-1)*k20*k5*x5/(k6 + x5) + 1*k20*k7*k8*1/2*(x4 + x8)/(k9 + k8*1/2*(x4 + x8)) + 1*k20*k10)/k20,
  diff(x6, t) = (1*k20*k11*x5 + (-1)*k20*k12*x6/(k13 + x6))/k20,
  diff(x7, t) = (1*k20*k14*x6 + (-1)*k20*k15*x7/(k16 + x7))/k20,
  diff(x8, t) = (1*k20*k17*x5 + (-1)*k20*k18*x8/(k19 + x8))/k20 }