A)x+y=2⇒x=2-y
xy=-15
2y-y²=-15
y²-2y-15=0
y1+y2=2 U y1*y2=-15
y1=5⇒x1=2-5=-3
y2=-3⇒x2=2+3=5
(-3;5);(5;-3)
б)2x+y=-1⇒y=-1-2x
x²+2y=3
x²-2-4x-3=0
x²-4x-5=0
x1+x2=4 U x1*x2=-5
x1=-1⇒y1=-1+2=1
x2=5⇒y2=1-10=-9
(-1;1);(5;-9)
в)x-y=1
x²-y²=7⇒(x-y)(x+y)=7⇒x+y=7
прибавим
2x=8
x=4
4-y=1
y=3
(4;3)