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1. \(\dfrac{7}{9ab}-\dfrac{13}{12ab}=\) [ \(-\frac {11}{36ab}\) | \(\frac {11}{ab}\) | \(-\frac{1}{36ab}\) | \(\frac {1}{ab}\) ];
2. \(\dfrac{2n-5m}{n}+\dfrac{6n^2+5m^2}{mn}=\) [ \(\frac {2(m+3n)}{m}\) | \(\frac {m+n}{n}\) | \(-\frac{3}{m+3n}\) | \(\frac {1}{m+3n}\) ];
3. \(6-\dfrac{3a+6c}{c}=\) [ \(-\frac {4}{a}\) | \(\frac ac\) | \(-\frac {3a}{c}\) | \(\frac 1a\) ];
4. \(\dfrac{n^2-m^2}{m+3n}+m-3n=\) [ \(\frac {1}{m}\) | \(\frac {m}{m+3n}\) | \(-\frac{8n^2}{m+3n}\) | \( \frac{5}{m+3n}\) ].