19.5: Comparison of construction tools
We say that one set of tools is stronger than another if any configuration of points that can be constructed with the second set can be constructed with the first set as well. If in addition, there is a configuration constructable with the first set, but not constructable with the second, then we say that the first set is stictly stronger than the second. Otherwise (that is, if any configuration that can be constructed with the first set can be constructed with the second) we say that the sets of tools are equivalent .
For example Mohr–Mascheroni theorem states that compass alone is equivalent to compass and ruler. Note that one may not construct a line with a compass, but since we consider only configurations of points we do not have to. One may think that a line is constructed , if we construct two points on it.
For sure compass and ruler form a stronger set than compass alone. Therefore Mohr–Mascheroni theorem will follow once we solve the following two construction problems:
- Given four points \(X\) , \(Y\) , \(P\) and \(Q\) , construct the intersection of the lines \((XY)\) and \((PQ)\) with compass only.
- Given four points \(X\) , \(Y\) , and a circle \(\Gamma\) , construct the intersection of the lines \((XY)\) and \(\Gamma\) with compass only.
Indeed, once we have these two constructions, we can do every step of a compass-and-ruler construction using compass alone.
Compare the following set of tools: (a) a ruler and compass, (b) a set-square, and (c) a ruler and a parallel tool.
- Hint
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A ruler and compass is strictly stronger than a set-square which is strictly stronger than a ruler and a parallel tool. To prove it, use Exercise 5.7.1 , Exercise 14.2.4 , Exercise 19.3.2 and Proposition 7.1.1 .
Another classical example is the so-called Poncelet–Steiner theorem ; it states that compass and ruler is equivalent to ruler alone, provided that a single circle and its centre are given.