When we are dealing with linear systems, the only way signals can be
combined is by scaling(multiplication of the signals by constants) followed by
addition. For instance, a signal cannot be multiplied by another signal. Figure
5-10 shows an example: three signals: , , and are added to form x
0
[n] x
1
[n] x
2
[n]
a fourth signal, . This process of combining signals through scaling and x [n]
addition is called synthesis.