Example case:
[tex]\begin{cases}x+y=1\\2x+2y=2\end{cases}[/tex]
The second equation is twice the first, so subtracting that from the second equation yields the identity 0 = 0.
This is an identity. It's an equation that holds true (clearly, 0 is the same as 0) regardless of the values of the variables, which means any pair of numbers [tex](x,y)[/tex] is a solution to the system.
