[tex]x=7n-11+3\cdot (-1)^{n+1} \\ y=7n+18-3 \cdot (-1)^n \\ \bold{a)} \ n=0 \\ \Rightarrow \ x=0-11+3\cdot (-1)=-11+(-3)=-14; \\ \Rightarrow \ y=0+18-3\cdot(-1)^0=18+3=21; \\ \Rightarrow \ x-y=-14-21=\boxed{-35};[/tex]
[tex]\bold{b)} \ pentru \ n=-2: \\ \frac{x}{y}=\frac{7\cdot(-2)-11+3\cdot (-1)^{-2+1}}{7\cdot(-2)+18-3\cdot (-1)^{-2}}=\frac{-14-11+3\cdot (-1)^{-1}}{-14+18-3\cdot(-1)^{-2}}= \\ \\ =\frac{-14-11+3\cdot (-1^{-1})}{-14+18-3 \cdot 1}=\frac{-14-11+3\cdot (-1)}{1} \ (deja \ divide)=-28.[/tex]
