I have three points, $A$, $B$, and $C$, on an ellipse; two tangent lines at $A$ and $B$ are also known. Are these enough to determine the whole ellipse?
2026-05-04 12:25:40.1777897540
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Is an ellipse determined by three points and tangent lines at two of them?
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A conic by five distinct points can be constructed as follows: consider the lines $d_{12}=0,d_{34}=0,d_{13}=0$ and $d_{24}=0$ and form the pencil of conics
$$(1-\lambda)d_{12}d_{34}+\lambda d_{13}d_{24}=0.$$ these are all the conics through $1,2,3,4$. Then plug the coordinates of $5$ in the equation and determine $\lambda$ to get the conic by $1,2,3,4,5$.
Now if $1$ and $2$ get closer and closer, $d_{12}$ tends to a tangent, let $t_1$, and the pencil becomes
$$(1-\lambda)t_{1}d_{34}+\lambda d_{13}d_{14}=0.$$
If $4$ also migrates to $3$, defining the tangent $t_3$,
$$(1-\lambda)t_{1}t_3+\lambda d_{13}^2=0.$$
Answered in comments by OP: