{ diff(x1, t) = ((-1)*k12*k6*x4*x1 + (-1)*k12*k8*x1 + 1*k12*k7)/k12,
  diff(x2, t) = ((-1)*k12*k4*x3*x2 + (-1)*k12*k2*x2 + 1*k12*k3)/k12,
  diff(x3, t) = (1*k12*k9*x4 + (-1)*k12*k10*x3 + (-1)*k12*k11*x3 + (-1)*k12*k4*x3*x2)/k12,
  diff(x4, t) = ((-1)*k12*k9*x4 + 1*k12*k10*x3 + (-1)*k12*k5*x4 + (-1)*k12*k6*x4*x1)/k12,
  diff(x5, t) = 1*k12*k11*x3/k12 }