Section construction9_2.
Variable t : Triangle.
Let C : Point := summit t.
Let A : Point := origin (base t).
Let B : Point := extremity (base t).
Let Base : Line := ln (base t).
Let L1 : Line := ln (Side1 t).
Let L2 : Line := ln (Side2 t).
Let L3 : Line := par Base C.
Let L4 : Line := par L1 B.
Let lemma1 : ConLn Base L1.
Hint Resolve lemma1.
Let lemma2 : DiLn Base L3.
Hint Resolve lemma2.
Let lemma5' : Apart B L1.
Hint Resolve lemma5'.
Let lemma2' : DiLn L1 L4.
Hint Resolve lemma2'.
Let lemma3 : ConLn L1 L3.
Hint Resolve lemma3.
Let lemma4 : ConLn L3 L4.
Hint Resolve lemma4.
Let D : Point := pt (Twol L3 L4 lemma4).
Let lemma5 : Apart B L3.
Hint Resolve lemma5.
Let lemma6 : Apart C L4.
Hint Resolve lemma6.
Let lemma7 : DiPt C D.
Let lemma8 : DiPt B D.
Hint Resolve lemma7 lemma8.
Let S1 : Segment := base t.
Let S3 : Segment := reverse (Side1 t).
Let S2 : Segment := Seg C D lemma7.
Let S4 : Segment := Seg B D lemma8.
Let lemma9 : EqLn L1 (ln (reverse (Side1 t))).
Hint Resolve lemma9.
Let lemma10 : EqLn L3 (ln S2).
Hint Resolve lemma10.
Let lemma11 : EqLn L4 (ln S4).
Hint Resolve lemma11.

Theorem thm9_2 : Parallelogram.