The Tangent Theorem If P is a point on the conic C, then there is a unique line L tangent to C at P.
Proof
By the affine transitivity theorem, there is an affine
transformation t which maps
If M is any tangent at P, t(M) is a tangent to C0 at T, so
t(M) = L, i.e. M = t-1(L).
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