1) {x^2+y^2=-21
{x-y=3
{(x-y)(x+y)=-21
{x-y=3
{3(x+y)=-21 | :3
{x+y=7
{x+y=-7
{x-y=3
{x=-7-y
{-7-2y=3
-7-2y=3
y=-5
x=-7+5
x=-2
Ответ: (-5;-2)
2) {x^2+y^2=74
{x-y=2
{x=2+y
{(2+y)^2+y^2=74
4+4y+2y^2=74
2y^2+2y-35=0
D=144
y1=5 y2=-7
Когда y=5, x=7
Когда y=-7, x=-5
Ответ: (7;5)∪(-5;-7)