Thực hiện phép tính $\frac{1}{x(x+y)}$+ $\frac{1}{y(x+y)}$+ $\frac{1}{x(x-y)}$+ $\frac{1}{y(y-x)}$

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1. `\text{~~Holi~~}`

   `1/(x(x+y))+1/(y(x+y))+1/(x(x-y))+1/(y(y-x))`

   `= 1/(x(x+y))+1/(y(x+y))+1/(x(x-y))-1/(y(x-y))`

   `= (y(x-y)+x(x-y)+y(x+y)-x(x+y))/(xy(x+y)(x-y))`

   `= (xy-y^2+x^2-xy+xy+y^2-x^2-xy)/(xy(x^2-y^2)`

   `= 0/(xy(x^2-y^2)`

   `= 0`

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