Помогите плизззззззз 3 и 5 упражнение

3. а) $\left \{ {{5x+y=11} \atop {x+y=7}} \right.$
   $\left \{ {{5x+y=11} \atop {-x-y=-7}} \right.$
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   $4x=4$
   $\left \{ {{x=1} \atop {1+y=7}} \right.$ $\left \{ {{x=1} \atop {y=6}} \right.$
б) $\left \{ {{5x-3y=11} \atop {3x+y=1}} \right.$
   $\left \{ {{5x-3y=11} \atop {9x+3y=3}} \right.$
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   $14x=14$
   $\left \{ {{x=1} \atop {3*1+y=1}} \right.$ $\left \{ {{x=1} \atop {y=-2}} \right.$
5. $\left \{ {{ \frac{4}{x-y}+\frac{12}{x+y}=3} \atop { \frac{18}{x+y}- \frac{8}{x-y}=1}} \right.$
   $\left \{ {{4s+12t=3} \atop {18t-8s=1}} \right.$
   $\left \{ {{8s+24t=6} \atop {-8s+18t=1}} \right.$
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   $42t=7$
   $t= \frac{7}{42}$
   $\left \{ {{t= \frac{1}{6} } \atop {4s+12* \frac{1}{6}=3}} \right.$ $\left \{ {t= \frac{1}{6}} \atop {s= \frac{1}{4} }} \right.$
   $\left \{ {{ \frac{1}{x-y}= \frac{1}{4}} \atop { \frac{1}{x+y}=\frac{1}{6}}} \right.$
   $\left \{ {{x-y=4} \atop {x+y=6}} \right.$
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   $2x=10$
   $\left \{ {{x=5} \atop {5-y=4}} \right.$ $\left \{ {{x=5} \atop {y=1}} \right.$