For
let CE be drawn through the point C parallel to the straight line AB. [I.31]
Then, since
AB is parallel to CE, and AC has fallen upon them,
the alternate angles BAC, ACE are equal to one another. [I.29]
Again, since
AB is parallel to CE, and BD has fallen upon them,
the exterior angle ECD is equal to the interior and opposite angle ABC. [I.29]

