Pascal's Thm
If a hexagon is inscribed in a conic,the three pairs of opposite sides intersect one another in three collinear points.
How to find the tangent line of ellipse by Pascal's Thm？
Let A&B',C',C,A' and B be five points in the ellipse,respectively. 
step 1:
Find the points Q and R as the following.
step 2:
The point P be the intersection point of the straight line through A,B' and the straight line through B,A'.
By Pascal's Thm,P is also on the straight line through Q,R.
So point P be the intersection point of the straight line through A,B' and the straight line through Q,R.
step 3:
Draw a straight line through A&B' and P.
Then the straight line through A&B' and P will be the tangent line of ellipse at point A&B'.
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