# Straightedge and compass constructions

This is a quick note about straightedge and compass constructions, in relation to my set.

A quick definition: a straightedge is an arbitrarily long straight edge, and a compass is a circle drawing tool. Specifically, with a straight edge you can:

• Pick any two lines and draw the line through them

• Pick a point and draw a line through it

• Draw an arbitrary line

And with a compass you can:

• Pick two points, $A,B$ and draw a circle centered at $A$ with radius $|AB|$

• Pick a point and draw a circle centered at that point

• Draw an arbitrary circle going through a point

• Draw an arbitrary circle

Just a quick note about a compass, assume in the questions that you are using a non-collapsing compass, that is, if you draw a circle then you can make your next circle have the same radius as the circle you constructed free of charge.

I'll call a geometric construction constructible if it can be constructed with a straightedge and compass. For example which shows that an equilateral triangle is constructible.

A constructible number, say $x$, is defined as follows:

If given a unit length, it is possible to construct a segment of length $x$.

Also a shape is constructible given another shape if it is possible to construct another shape given the first one. So although a $22$-gon is not constructible, it is constructible given an $11$-gon.

Lastly a use is a single use of a compass or straightedge, as defined above. So the equilateral triangle takes 5 moves.

Again this is in relation to my set. Note by Wen Z
3 years, 4 months ago

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