Решить систему уровнений

$\left \{ {{x+y=7} \atop {x^{2}-9y=7}} \right. y=7-x x^{2} -9(7-x)=7 x^{2} -63+9x=7 x^{2} +9x-70=0 D=9^{2} -4*1*(-70)=81+280=361 x_{1}= \frac{-9-19}{2*1}= \frac{-28}{2}=-14 x_{2}= \frac{-9+19}{2*1}= \frac{10}{2}=5 -14+y=7 y=21 5+y=7 y=2$
Ответ:(-14;21) и (5;2)

$\left \{ {{xy=12} \atop { \frac{3}{x}- \frac{1}{y}= \frac{5}{12} }} \right. \left \{ {{y= \frac{12}{x} } \atop { \frac{2}{x}- \frac{1}{y}= \frac{5}{12} }} \right. \left \{ {{y= \frac{12}{x} } \atop { \frac{2}{x}- \frac{x}{12}= \frac{5}{12} }} \right. 24- x^{2}=5x - x^{2}-5x+24=0 x^{2} +5x-24=0 D=5^{2} - 4*1*(-24)=25+96=121 x_{1}= \frac{-5-11}{2*1}= \frac{-16}{2}=-8 x_{2}= \frac{-5+11}{2*1}= \frac{6}{2}=3 y_{1}= \frac{12}{-8}=-1.5 y_{2} = \frac{12}{3}=4$
Ответ: (-8;3) и (3;4)