Hyperbolas with prescribed asymptotes ($L_1(x,y)L_2(x,y)=c$)

For two distinct intersecting straight lines through the origin, with independent real linear equations $L_1=0$ and $L_2=0$, the nondegenerate centred hyperbolas having exactly those asymptotes areIndeed, the homogeneous quadratic form of a centred hyperbola vanishes in its two asymptotic directions and hence is a nonzero scalar multiple of $L_1{L}_2$. Its equation has no linear terms because the origin is its centre, and its constant term is nonzero by nondegeneracy. Conversely, the invertible linear transformation $(x,y)\mapsto (L_1,L_2)$ takes the displayed curve to $uv=c$, a hyperbola, and its ends approach $u=0$ or $v=0$. Distinct values of $c$ give disjoint level sets. In particular, the rectangular hyperbolas with asymptotes $y=\pm x$ are $x^2-y^2=c$, $c\ne 0$.