To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x+y$ and $x-y$ is $\left(x+y\right)\left(x-y\right)$. Multiply $\frac{x-y}{x+y}$ times $\frac{x-y}{x-y}$. Multiply $\frac{x+y}{x-y}$ times $\frac{x+y}{x+y}$.
Since $\frac{\left(x-y\right)\left(x-y\right)}{\left(x+y\right)\left(x-y\right)}$ and $\frac{\left(x+y\right)\left(x+y\right)}{\left(x+y\right)\left(x-y\right)}$ have the same denominator, subtract them by subtracting their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x+y$ and $x-y$ is $\left(x+y\right)\left(x-y\right)$. Multiply $\frac{x-y}{x+y}$ times $\frac{x-y}{x-y}$. Multiply $\frac{x+y}{x-y}$ times $\frac{x+y}{x+y}$.
Since $\frac{\left(x-y\right)\left(x-y\right)}{\left(x+y\right)\left(x-y\right)}$ and $\frac{\left(x+y\right)\left(x+y\right)}{\left(x+y\right)\left(x-y\right)}$ have the same denominator, subtract them by subtracting their numerators.
