1/x^2+1/y^2=9/64
(x^2+y^2)/x^2*y^2=9/64
xy=8
(x^2+y^2)/x^2*y^2=9/64
(x^2+y^2)/64=9/64
x^2+y^2=9
x^2+y^2+2xy=9+16
(x+y)^2=25
x+y=5,-5