Tvrdenie XVI. 
V každom trojuholníku, ktorého jedna strana sa predĺži, vonkajší uhol je väčší ako ktorýkoľvek protiľahlý vnútorný uhol.

Stred(A, C)

E

φ

φ - vrcholový

Δ ABE ≅ΔFEC

⇒

∡BAE≅∡FCE

⇒

∡BAE<∡ACD

A = (-4, 2)
B = (-8, -6)
C = (6, -6)
t1, c, a, b = Polygon(A, B, C)
h = Ray(B, C)
D = Point(h)
E = Midpoint(A, C)
G: E
g = Ray(B, E)
d = Circle(E, B)
f = Segment(B, E)
F = Intersect(d, g, 2)
k = Segment(E, F)
β = Angle(A, E, B)
γ = Angle(F, E, C)
t2, e, a_1, b_1 = Polygon(A, B, E)
t3, c_1, f_1, e_1 = Polygon(F, E, C)
α = Angle(B, A, E)
δ = Angle(E, C, F)
ε = Angle(A, C, D)