Đáp án: a)$\left \{ {{x²-y²=16} \atop {x-y=2}} \right.$
⇔$\left \{ {{x²-y²=16} \atop {x=2+y}} \right.$
⇔$\left \{ {{(2+y)²-y²=16} \atop {x=2+y}} \right.$
⇔$\left \{ {{(2+y-y)(2+y+y)=16} \atop {x=2+y}} \right.$
⇔$\left \{ {{2.(2+2y)=16} \atop {x=2+y}} \right.$
⇔$\left \{ {{2+2y=8} \atop {x=2+y}} \right.$
⇔$\left \{ {{2y=6} \atop {x=2+y}} \right.$
⇔$\left \{ {{y=3} \atop {x=2+3}} \right.$
⇔$\left \{ {{y=3} \atop {x=5}} \right.$
b)$\left \{ {{\frac{4x+5y}{xy}=2 } \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{4x+5y=2xy} \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{4x+5y-2xy=0} \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{4x+5y-2xy=0} \atop {40x-60y+2xy=0}} \right.$
⇔$\left \{ {{44x-55y=0} \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{11(4x-5y)=0} \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{4x-5y=0} \atop {20x-30y+xy=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {20.\frac{5y}{4}-30y+\frac{5y}{4}y=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {\frac{100y}{4}-30y+\frac{5y²}{4}y=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {\frac{-20y+5y²}{4}=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {-20y+5y²=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {5y(-4+y)=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {-4+y=0}} \right.$
⇔$\left \{ {{x=\frac{5y}{4} } \atop {y=4}} \right.$
⇔$\left \{ {{x=\frac{5.4}{4} } \atop {y=4}} \right.$
⇔$\left \{ {{x=5 } \atop {y=4}} \right.$
END =))
