А2. способ сложения
{х-7у=20 ||×(-5)
{5х+2у=26
{-5х+35у=-100
{5х+2у=26
5х-5х+2у+35у=26-100
37у=-74
у=-2
х=20+7у=20+7×(-2)=6
х+у=6-2=4
а3.
{3х-10у=1 ||×(-3)
{9х+2у=67
{-9х+30у=-3
{9х+2у=67
9х-9х+30у+2у=67-3
32у=64
у=2
3х-10×2=1
3х-20=1
3х=21
х=7
х-у=7-2=5