[tex]\displaystyle\bf\\II)\\1)\\a+2b=7\\4b+3c=19\\5a+22b+9c=?\\............\\a+2b=7~~\Big|\times 5\\~~~~~~4b+3c=19~~\Big|\times3\\......................................\\5a+10b=35\\\underline{~~~~~~~12b+9c=57}\\5a+22b+9c=92\\\\\\2)\\\\x=5y\implies~\frac{x}{y}=5~~si~~\frac{y}{x}=\frac{1}{5}\\\\\frac{x}{y}-\frac{y}{x}=5-\frac{1}{5}=\frac{5}{1}-\frac{1}{5}=\frac{25}{5}-\frac{1}{5}=\frac{25-1}{5}=\frac{24}{5}\\\\\\[/tex]
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[tex]\displaystyle\bf\\3)\\\\3a+b=5\\5b-4c=11\\12a-11b+12c=?\\..................................\\3a+b=5~~\Big|~\times4\\~~~~~~~5b-4c=11~~\Big|~\times(-3)\\...............................................\\12a+4b=20\\\underline{~~~~~~-15b+12c=-33}\\12a-11b+12c=-13[/tex]
