Linear System
A system is called "Linear" if for any constants a,b and for any inputs x1(t),x2(t),(x1[n],x2[n]) yielding output y1(t),y2(t),respectively, the respond to a*x1(t)+b*x2(t) is a*y1(t)+b*y2(t)
Example
Let:
x1(t)=t, x2(t)=2*t;
y1(t)=2*x1(t)= 2*t, y2(t)=3*x2(t)= 6*t;
a=2, b=3;
so, a*x1(t)+b*x2(t)=
