X^2 - 2x - 3 = x^2 - 2x + 1 - 4 = (x-1)^2 - 2^2 = (x-1 -2)*(x-1+2) = (x-3)*(x+1).
x^2 + 4x -5 = x^2 + 2*2x + 4 - 9 = (x+2)^2 - 3^2 = (x+2-3)*(x+2+3) = (x-1)*(x+5).
x^2 + 6x - 7 = x^2 + 2*3x + 9 - 9 - 7 = (x+3)^2 - 16 = (x+3-4)*(x+3+4)=
= (x-1)*(x+7).
x^2 - 8x - 9 = x^2 - 2*4x + 16 - 16 - 9 = (x-4)^2 - 25 = (x-4-5)*(x-4+5)=
= (x-9)*(x+1).
2х^2-у^2=34
xy=20
y=20/x
2x^2-(20/x)^2=34
2x^2-400/x^2-34=0
2x^4-34x^2-400=0
Замена: x^2=t
2t^2-34t-400=0
t^2-17t-200=0
D=289+800=1089
t1=(17+33)/2=25
t2=(17-33)/2=-8 - не подходит
Обратная замена:
x^2=25
1) x=5, y=4
2) x=-5, y=-4