Solve the system of differential equations using the Runge-Kutta method and NDSolve, display the solution in one graph.
Use ParametricPlot (for NDSolve solution) to create a graph of the dependence of x2 on x1 and ListPlot (Runge-Kutta solution) and display them in one graph.
For all graphs, describe the axes and adjust the PlotStyle. 
exns={x1’(t)=x2(t)-0.1x1(t),x2’(t)=cos(3t)-3x2(t)-x1(t),x1(0)=0,x2(0)=1},
nSteps=25;
tmax=10

Use NMinimize and NMaximize to find the minimum and maximum of the constraint expression.
Display found points and expression graph using Plot3D and ListPlot3D in one graph. 
expr=0.1*((x^2)+2x*y+(y^2))+x*sin(x+y)+y*Cos(x-y);
constr=((((x/1.5)))^2)+((((y/2.5)))^2)<1.