[ Diff(x[1], t) = (1*k[9]*x[2]/k[9] + (-1)*k[9]*k[1]*x[1]/k[9])/k[9],
  Diff(x[2], t) = (1*k[9]*k[5]*piecewise(t < k[7]*Pi, sin(t)^2, 0)*piecewise(t < k[8], 1, 0)/k[9] + 1*k[9]*k[2]*x[3]^2/(k[6]^2 + x[3]^2)/k[9] + (-1)*k[9]*x[2]/k[9])/k[9],
  Diff(x[3], t) = (1*k[9]*k[1]*x[1]/k[9] + (-1)*k[9]*k[2]*x[3]/k[9] + (-1)*k[9]*k[3]*x[4]*x[3]/k[9])/k[9],
  Diff(x[4], t) = (1*k[9]*x[2]/k[9] + (-1)*k[9]*k[4]*x[4]/k[9])/k[9] ]