Frégier's theorem states that if a right angle with vertex 
on a nondegenerate conic section meets the conic
section again at 
and 
,
then the line 
passes through a fixed point 
in the projective
plane, independent of the right angle, as illustrated
above.
For a rectangular hyperbola, the hypotenuse lines 
are parallel, so their common
point 
is a point at infinity
(Prisacariu 2022).
