If a straight line stood on a(nother) straight line makes angles, it will certainly either make two right angles, or angles whose sum is) equal to two right angles. * For let some straight line $AB$ stood on the straight line $CD$ make the angles $CBA$ and $ABD$. * I say that the angles $CBA$ and $ABD$ are certainly either two right angles, or (have a sum) equal to two right angles.
If the straight line \(AB\) intersects the straight line \(CD\) at one and only one point (\(B\)), then either \(\angle{ABC}\) and \(\angle{DBA}\) are right angles or the sum \(\angle{ABC}+\angle{DBA}\) equals the sum of two right angles.
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