{ diff(x1, t) = (-1)*k1*k2*(x1 - x2)/piecewise(500, t <= k12, 400),
  diff(x2, t) = (k1*k2*(x1 - x2) - k1*k2*(x2 - x3) - k3*x2*piecewise(33/100, t <= k12, 33/100) - k4*x2*piecewise(33/100, t <= k12, 33/100))/piecewise(33/100, t <= k12, 33/100),
  diff(x3, t) = (k1*k2*(x2 - x3) - k7*x3*piecewise(380, t <= k12, 280) - k11*x3*piecewise(380, t <= k12, 280))/piecewise(380, t <= k12, 280),
  diff(x4, t) = (-1)*k9*k2*(x4 - x5)/piecewise(500, t <= k12, 400),
  diff(x5, t) = (k9*k2*(x4 - x5) - k9*k2*(x5 - x6) + k3*x2*piecewise(33/100, t <= k12, 33/100))/piecewise(33/100, t <= k12, 33/100),
  diff(x6, t) = (k9*k2*(x5 - x6) + k7*x3*piecewise(380, t <= k12, 280))/piecewise(380, t <= k12, 280),
  diff(x7, t) = (-1)*k10*k2*(x7 - x8)/piecewise(500, t <= k12, 400),
  diff(x8, t) = (k10*k2*(x7 - x8) - k10*k2*(x8 - x9) + k4*x2*piecewise(33/100, t <= k12, 33/100))/piecewise(33/100, t <= k12, 33/100),
  diff(x9, t) = (k10*k2*(x8 - x9) + k11*x3*piecewise(380, t <= k12, 280))/piecewise(380, t <= k12, 280) }