Théorème de Céva 
Les droites (aa'), (bb') et (cc') sont concourantes 
si et seulement si on a :
[v,w,u,u'] x [w,u,v,v'] x [u,v,w,w']=-1

a = (-2.5028187919463103, -1.3122938530734631)
b = (-3.08268456375839, -2.533163418290854)
a_1 = Line(a, b)
c = (-1.7876510067114115, -2.7954722638680654)
b_1 = Line(c, b)
c_1 = Line(a, c)
a' = Point(b_1)
b' = Point(c_1)
d = Line(a, a')
e = Line(b, b')
F = Intersect(d, e)
f = Line(c, F)
c' = Intersect(a_1, f)
w = Point(a_1)
v = Point(c_1)
g = Line(w, v)
u' = Intersect(d, g)
v' = Intersect(e, g)
w' = Intersect(f, g)
u = Intersect(b_1, g)