When constructing perpendicular lines with a compass and straightedge, how should you start the construction?
Open the compass and mark two points of intersection between arcs from the given line.
This would be a very sensible way to start the construction of a perpendicular. Next we would make same radii arcs from each of the intersection points and connect their two meets to construct the perpendicular bisector.
Use a straightedge to find the length of the given line and then make an arc above it.
No, there isn't any measuring allowed with a classic straightedge; it's not a ruler.
Open the compass to the width of the original line to measure it.
Again no measuring in classic straightedge/compass constructions, though we usually allow copying of lengths like this with a compass as a shortcut to a more formal length length copy construction.
Use a straightedge to create two arcs above and below the original line.
Straightedge to create arcs, is that a joke?
Answer: Open the compass and mark two points of intersection between arcs from the given line.
