To construct the perpendicular bisector of a line AB with a compass and straight edge, follow these steps.

- Put the compass point on A.
- Open the compass to about 2/3 of the length AB.
- Make one big arc from top to bottom which cuts through AB.
- Put the compass point on B.
- Make another big arc from top to bottom which cuts through AB.
- You should now have two arcs that cut across AB, but also cross each other at two points, C and D.
- Draw the line CD. It should be perpendicular to AB and bisect it.

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