[tex] \left \{ {{2x+3y=21|*2\ \textless \ =\ \textgreater \ 4x+6y=42} \atop {7x-2y=11}|*3\ \textless \ =\ \textgreater \ 21x-6y=33} \right. \\
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25x=75\\
x=3\\\\
2*3+3y=21=\ \textgreater \ 6+3y=21=\ \textgreater \ 3y=15=\ \textgreater \ y=5\\\\
a) \frac{x}{y} = \frac{3}{5} \\
b) \frac{x+y}{x-y} = \frac{8}{-2} \\\\
Proba: \frac{2x+3y}{7x-2y} = \frac{21}{11} \\
=\ \textgreater \ (2*3+3*5)*11=(7*3-2*5)*21\\
=\ \textgreater \ (15+6)*11=(21-10)*21\\
=\ \textgreater \ 21*11=11*21\\
=\ \textgreater \ 231=231[/tex]
