{ diff(x1, t) = (1*k6*1/4*k5*(x2 + x3)*x1 + (-1)*k6*1/k3*x1)/k6,
  diff(x2, t) = (1*k6*k2*x2 + (-1)*k6*k5*x2*x1)/k6,
  diff(x3, t) = (-1)*k6*(k5*x3*x1 + 1/k1*x3)/k6 }