Feature Column

Visual Explanations in Mathematics


4. Proofs of ancillary facts

1. In a regular pentagon, the ratio of diagonal to side is the Golden Mean 1.618033...

Proof: Use the notation from the incommensurability proof. The ratio we are interested in is d/s. This ratio must be the same in the smaller regular pentagon, where it becomes d*/s*. Substituting into the equation d/s = d*/s* the expressions for d* and s* in terms of d and s yields:

          d        d - s         ---  =  -------- .          s       2s - d 

Multiplying out and regrouping gives:

          d2 - sd - s2 = 0. 

Dividing through by s2 leaves us with:

              d2      d         ---  -  ---  - 1 = 0.          s2      s 

So d/s is the positive root of the equation x2 - x - 1 = 0, i.e. the Golden Mean.

2. A number is rational if and only if it is commensurable with 1.

Proof: If a number x is commensurable with 1, that means there exists a number h which is contained exactly a whole number of times in 1 and in x. Suppose it is contained q times in 1 and p times in x. Then h = 1/q and x = p/q, so x is rational. Conversely if x is rational, say x = p/q with p and q integers, then taking h = 1/q shows that x and 1 are commensurable.

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