Question

найти сумму корней уравнений {3x-7y=8 3/7 4x+2y=-1 5/7

Answer 1

$\displaystyle\begin{cases}3x-7y=8\frac{3}{7}|*4\\4x+2y=-1\frac{5}{7}|*3\end{cases}\to\begin{cases}12x-28y=\frac{236}{7}|*4\\-\\12x+6y=-\frac{36}{7}|*3\end{cases}\\-34y=\frac{272}{7}\\y=-\frac{8}{7}\\3x+8=8\frac{3}{7}\\3x=\frac{3}{7}\\x=\frac{1}{7}\\x+y=-\frac{8}{7}+\frac{1}{7}=-1$