If X1(t) -> system => y1(t) and X2(t) -> system => y2(t) implies a*X1(t)+ b*X2(t) -> system => a*y1(t) + b*y2(t)
for any complex number a,b
then the system is called linear.
E.g.
y(t)=$ x^2 (t) $ is non-linear
Since a*y1(t) + b*y2(t)=$ (a*X1(t)+ b*X2(t))^2= (a*X1(t))^2+ 2*(a*X1(t)*(b*X2(t)) + (b*X2(t))^2 $
y(t)=2(x)t is linear
Since a*y1(t) + b*y2(t)=2*(a*X1(t)+ b*X2(t))= 2*a*X1(t)+ 2*b*X2(t)
