If two straight lines, not lying on the same side, make adjacent angles (whose sum is) equal to two right angles with some straight line, at a point on it, then the two straight lines will be straight-on (with respect) to one another. * For let two straight lines $BC$ and $BD$, not lying on the same side, make adjacent angles $ABC$ and $ABD$ (whose sum is) equal to two right angles with some straight line $AB$, at the point $B$ on it. * I say that $BD$ is straight-on with respect to $CB$.
If at the endpoint of a ray \(\overline BA \) there exist two other rays \(\overline BC \), \(\overline BD \) standing on opposite sides of that ray such that the sum of their adjacent angles is equal to two right angles $\angle{DBA} + \angle{ABC}=2\cdot 90^\circ,$ then these two rays build a straight line \(CD \).
Proofs: 1
Proofs: 1 2 3 4 5 6 7
Propositions: 8
