|Book I Proposition 12|
let a point D be taken at random on the other side of the straight line AB,
and with center C and distance CD
let the circle EFG be described; [Post.3]
let the straignt line EG be bisected at H, [I.10]
let the straight lines CG, CH, CE be joined. [Post.1]
I say that CH has been drawn perpendicular to the given infinite straight line AB from the given point C which is not on it.
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