x^2-y^2=18

{

xy=9.

X=9/y

81-y^4=18y^2

y^4+18y^2+81=162

(y^2+9) ^2=9^2*2

y^2=9 * (sqrt (2) - 1)

x^2=9 * (sqrt (2) + 1)

Решения:

х=3*sqrt (sqrt (2) + 1))

y=3*sqrt (sqrt (2) + 1))

или

х=-3*sqrt (sqrt (2) + 1))

y=-3*sqrt (sqrt (2) + 1))