Tính các tổng sau:

\(\frac{5}{{6{x^2}y}} + \frac{7}{{12x{y^2}}} + \frac{{11}}{{18xy}}\);   

\(\frac{{5x + {y^2}}}{{{x^2}y}} - \frac{{5y - {x^2}}}{{x{y^2}}}\)

\( = \frac{{y\left( {5x + {y^2}} \right)}}{{{x^2}{y^2}}} - \frac{{x\left( {5y - {x^2}} \right)}}{{{x^2}{y^2}}}\) (Mẫu thức chung là: x²y²)

\( = \frac{{5xy + {y^3}}}{{{x^2}{y^2}}} - \frac{{5xy - {x^3}}}{{{x^2}{y^2}}}\)

\( = \frac{{5xy + {y^3} - 5xy + {x^3}}}{{{x^2}{y^2}}} = \frac{{{x^3} + {y^3}}}{{{x^2}{y^2}}}\).

Ta có \(\frac{{2x - 6}}{{{x^3} - 3{x^2} - x + 3}}\)\( = \frac{{2\left( {x - 3} \right)}}{{\left( {{x^3} - 3{x^2}} \right) - \left( {x - 3} \right)}}\)

\( = \frac{{2\left( {x - 3} \right)}}{{{x^2}\left( {x - 3} \right) - \left( {x - 3} \right)}}\)\( = \frac{{2\left( {x - 3} \right)}}{{\left( {x - 3} \right)\left( {{x^2} - 1} \right)}}\)\( = \frac{2}{{{x^2} - 1}}\).