1)(x+y)/(x-y)+x/y=(xy+y²+x²-xy)/y(x-y)=(x²+y²)/y(x-y)
2)(x+y)/(x-y)-y/x=(x²+xy-xy+y²)/x(x-y)=(x²+y²)/x(x-y)
3)(x²+y²)/y(x-y) *(x²+y²)/x(x-y)=(x²+y²)²/xy(x-y)²
4)(x²+y²)²/(x²-y²)² : (x²+y²)²/xy(x-y)²= (x²+y²)²/(x-y)² (x+y)² *xy(x-y)/(x²+y²)²=xy/(x+y)²