[tex]\displaystyle 5 < x < 6; 7 < y < 8\\\\ -8 < -y < -7; \frac{1}{8} < \frac{1}{y} < \frac{1}{7}[/tex]

теперь решение

[tex]\displaystyle 5+7 < x+y < 6+8\\\\12 < x+y < 14[/tex]

[tex]\displaystyle5+(-8) < x+(-y) < 6+(-7)\\\\-3 < x-y < -1[/tex]

[tex]\displaystyle 5*7 < x*y < 6*8\\\\35 < x*y < 48[/tex]

[tex]\displaystyle 5*\frac{1}{8} < x*\frac{1}{y} < 6*\frac{1}{7}\\\\\frac{5}{8} < \frac{x}{y} < \frac{6}{7}[/tex]