To place a straight line equal to a given straight line with one end at a given point.
Let A be the given point, and BC the given straight line.
We want to construct a line from point A that has the same length as line BC.
Join A to B.
Construct equilateral triangle DAB on AB (Prop. 1).
Produce DA and DB to E and F.
With center B and radius BC, describe circle CGH.
With center D and radius DG, describe circle GKL.
AL equals BC.