Find real parameter $a$, such that the solution of the linear system lies in the second quadrant 
For which real parameter $a$ lies the solution of the system of equations
$$\begin{aligned} \frac{x}{a+1} + \frac{y}{a-1} &= \frac{1}{a-1}\\ \frac{x}{a+1} - \frac{y}{a-1} &= \frac{1}{a+1} \end{aligned}$$
in the second quadrant?

I do not how to start to solve this system of equations. Any help?

Adding and subtracting the equations we obtain
$$\frac{2x}{a+1}  = \frac1{a-1}+\frac1{a+1}\implies x=\frac12 \frac{a+1}{a-1}+\frac12=\frac a{a-1}$$
$$\frac{2y}{a-1}  = \frac1{a-1}-\frac1{a+1}\implies y = \frac12-\frac12 \frac{a-1}{a+1}=\frac 1{a+1}$$
then we need $y>0$ and $x<0$.
