Bergo 2)
x^2+y^2=34
x+y=8

x^2+y^2=34
x=8-y

(8-y)^2+y^2=34
64-16y+y^2+y^2-34=0
2y^2-16y+30=0 
y^2-8y-15=0
Д=64-4*1*15=64-60=4  (2)
y1=8+2/2=5
y2=8-2/2=3
x1=8-5=3
y2=8-3=5
Ответ: x1=3, y1=5
           x2=5, y2=3

4)
x-y=12
3/5x=y^2

x=12+y
3/5x-y^2=0

3/5(12+y)-y^2=0
36/5+3/5y-y^2=0
36+3y-5y^2/5=0
5y^2-3y-36=0
Д=9-4*5*(-36)=9+270=279 (27)
y1=3+27/10=3
y2=3-27/30=-24/10=-2.4
x1=12+3=15
x2=12-2.4=9.6
Ответ: x1=15, y2=3
           x2=9.6, y2=-2.4