{ diff(x1, t) = (1*k9*k1*(1 + piecewise(13/5, and(t > 10, t < 150), 0) + k5*(x2/k6)^4/(1 + (x2/k6)^4)) + (-1)*k9*k2*x1*k8)/k9, diff(x2, t) = (1*k9*k3*x1*k8 + (-1)*k9*k4*x2*k8)/k9 }