Lösa system av ODE

x’(t) = -y(t) - x^2(t),       y’(t) = 2x(t) - y(t)^3 ,     x(0) = 1, y(0) = 1, lös för tid 0 <= t <= 20

sol = NDSolve[{x'[t]== -y[t]-x[t]^2, y'[t] == 2x[t]-y[t]^3, x[0]== 1, y[0] == 1},{x,y},{t,0,20}]

plot1 = Plot[Evaluate[x[t]/.sol],{t,0,20}]

plot2 = Plot[Evaluate[y[t]/.sol],{t,0,10}]

Show[plot1,plot2]

plot3 = ParametricPlot[Evaluate[{x[t],y[t]}/.sol],{t,0,10}]