Every operation that can be carried out with a straightedge and compass can be done with a compass alone.
Notes: [list] [*]d²=b² + 2h² : [url]http://www.geogebratube.org/material/show/id/31406[/url] (The diagonals of a trapezoid. In this problem the trapezoid is isosceles and the base is twice as long as the top.) [/list]