f = 2 (slider from -5 to 5)
F1: (f, f)
F2: ((-f), (-f))
k = 4 (slider from -5 to 20)
eq1:=sqrt((x + f)^(2) + (y + f)^(2)) - sqrt((x - f)^(2) + (y - f)^(2)) = k
eq2:=abs(sqrt((x + f)^(2) + (y + f)^(2)) - sqrt((x - f)^(2) + (y - f)^(2))) = k
k' = 2 (slider from -5 to 20)
g(x) = k' / x