4) вот написала )) спасибо :D

Решите задачу:

$\left \{ {{\frac{5}{m}-\frac{3}{n}}=\frac{7}{mn}} \atop {\frac{1}{m}+\frac{2}{n}=\frac{17}{mn}}} \right. \; \left \{ {{\frac{5n-3m}{mn}=\frac{7}{mn}} \atop {\frac{n+2m}{mn}=\frac{17}{mn}}} \right. \; \left \{ {{5n-3m=7} \atop {n+2m=17}} \right. \; \left \{ {{5(17-2m)-3m=7} \atop {n=17-2m}} \right. \; \left \{ {{13m=78,m=6} \atop {n=17-12=5}} \right.$