Tangent on Ellipse
Author: M. Dabbs, R. Walcott

Drag the point P.  Move the points A, S1 and S2

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S_1 = Point(a)
S_2 = Point(a)
O = Midpoint(S_1, S_2)
A = Point(a)
c = Circle(O, A)
d = Circle(O, S_2)
G = Point(c)
b = Line(S_1, G)
S'_1 = Dilate(S_1, -1.0, G)
e = Line(S_2, S'_1)
f = OrthogonalLine(G, b)
P_1 = Intersect(e, f)
loc1 = Locus(P_1, G)
P = Point(loc1)
g = Segment(S_1, P)
M = Midpoint(S_1, P)
h = Line(O, M)
C, D = Intersect(c, h)
i = Line(P, D)