### To erect a perpendicular from a point on a line

Given a point A on a line

scribe two arcs of the same radius centered at A intersecting the
line at two points on either side of A.

Call these two points B and C. Open the compass up a little more
than it already is and scribe arcs centered at B and C with the same
radius.

Call the point where the two arcs meet D. Draw AD.

AD is the line through A perpendiuclar to BC.

Proof

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