Question

Решите систему уравнений
$\left \{ {{15x-7y=8} \atop {20x+5y=-9}} \right.$

Answer 1

{15x-7y=8/*5⇒75x-35y=40
{20x+5y=-9/*7⇒140x+35y=-63
прибавим
215x=-23
x=-23/215
20*(-23/215)+5y=-9
-92/43+5y=-9
5y=-9+92/43
5y=-295/43
y=-295/43:5
y=-59/43
(-23/215;-59/43)