(21) 25gr. (40) AOV=OVD (81) ergoOVD†VOB=180 gr. (22) q. c. 1.Jam AOV+VOB=180. gr. (40)VOB=OVC (81) igitur AOVOVC=180 gr. (22) q. c. 2.85. THEOR. Si duo ad eandem par-tem interni anguli AOV†OVC duosadaequant rectos; sunt AB & CD paral-lelæ.DEMONST. OVD†OVC=180gr. (40) igitur si AOV†OVC=180gr., erit OVD†OVG=AOVtOVC(51) adeóque OVD=AOV(21) pro-inde sunt AB & CD parallelae (82) q. e. d.86. DEF. Si rectas AB & CD secatrecta EL, externi anguli vocantur EOB,EOA, LVC, LVD, qui nempe for-mantur extra lineas sectas. Ex his qui-libet opponi dicitur interno illi, cui non estdeinceps positus, & qui ad idem secantislatus existit. Nimirum externus LVDinterno VOB ad idem latus opponi-tur &c.87. THEOR. St parallelat AB &CD secat recta EL; augulus externusæqualis eſt interno ad idem latus oppoſito.DEMONST. LVD†OVD= 180gr. (40) VOB+OVD = 180 gr. (84)ergo LVD†OVD#VOB+OVD(51) adeóque LVD=VOB (21) Eo-dem modo idem ostendam de quovis alioexterno & opposito interno. q. c. d.B 288. Thron