Hint Resolve thm4_1c thm4_1d.

Theorem thm5_5 :
forall (a b c : Point) (H1 : DiPt a b) (H2 : EqPt b c),
EqLn (ln (Seg a b H1)) (ln (Seg a c (cong_eqpt_dipt a b c H1 H2))).