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If, at a point in a straight line, two other straight lines, upon the opposite sides of it, make the adjacent angles together equal to two right angles, these two straight lines shall be in one and the same straight line.
A companion to Euclid: being a help to the understanding and remembering of ... - Page 18
by Euclides - 1837 - 88 pages

The Elements of Plane Geometry

Euclid - 1845 - 336 pages
...quadrilateral such that BC = AD and AC = BD ; if AC, BD meet in O, show that OA = OB. 39. PROP. 1 2. If, at a point in a straight line, two other straight...adjacent angles together equal to two right angles, those two straight lines are in the same straight line. c B 'E Let BC, BD be two straight lines drawn...

A First Course of Geometry

130 pages
...circle centre O, and D is the mid-point of AB ; show that OD is perpendicular to BC. 25. PROP. 12. If, at a point in a straight line, two other straight...adjacent angles together equal to two right angles, those two straight lines are in the same straight line. Let BC, BD be two straight lines drawn from...