A system is called linear if for any inputs, x1 & x2, yielding outputs y1 & y2 the response to
a*x1 + b*x2 is a*y1 + b*y2.
i.e
The system below
x1 => system => *a
+ => y(t)
x2 => system => *b
equals th system below
x1*a => system
+ => y(t)
x2*b => system
