|Book II Proposition 10|
a straight line EF falls on the parallel straight lines EC, FD,
the angles CEF, EFD are equal to two right angles; [I.29]
the angles FEB, EFD are less than two right angles.
But straight lines produced from angles less than two right angles meet; [I. Post.5] therefore
EB, FD, if produced in the direction B, D, will meet.
Let them be produced and meet at G,
let AG be joined.
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