To bisect a given finite straight line.
Let AB be the given finite straight line.
Thus it is required to bisect the given straight line AB.
For, since AC is equal to
CB, and CD
the two sides AC, CD are equal to the two sides BC, CD respectively;
and the angle ACD is equal to the angle BCD;
therefore the base AD is equal to the base BD; [I. 4]
Therefore the given finite straight line AB has been bisected at D. Q.E.F.
Book I: Euclid, Book I (ed. Sir Thomas L. Heath 1st Edition, 1908)
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