c) Direction Field

> ode1 := diff( x(t), t ) = -x(t) - 2*y(t) :

> ode2 := diff( y(t), t ) = x(t) - 4*y(t) :

> MODEL := [ ode1, ode2 ] ;

> VAR := { x(t), y(t) } ;

>

> DOMAIN := t = 0 .. 1 ;

> WINDOW := x = -3 .. 3,

> y = -3 .. 3 ;

>

> IC := [ [x(0)= 3,y(0)= 3],

> [x(0)=-3,y(0)=-3],

> [x(0)= 3,y(0)= 1.5],

> [x(0)=-3,y(0)=-1.5] ];

> PHASEplot := DEplot( MODEL, VAR, DOMAIN, WINDOW, IC, arrows = MEDIUM,

> title = `Direction Field & Straight-Line Solutions for #10` ):

> PHASEplot;