2150.81 – A Horrendous Quantity


Simplify:

N=5+2+525+1+322.N = \frac{\sqrt{\sqrt{5} + 2} + \sqrt{\sqrt{5} - 2}}{\sqrt{\sqrt{5} + 1}} + \sqrt{3-2 \sqrt{2}}.

That is, find out what NN is.


Solution

Here is the horrendous thing:

N=5+2+525+1+322=km.N = \frac{\sqrt{\sqrt{5} + 2} + \sqrt{\sqrt{5} - 2}}{\sqrt{\sqrt{5} + 1}} + \sqrt{3-2 \sqrt{2}} = k - m.

We will deal with the two terms separately. The first term, which we call kk, we will square:

k2=(5+2+525+1)2=(5+2)+2(5+2)(52)+(52)5+1)=25+2545+1=25+25+1=2. \begin{aligned} k^2 &=&\left( \frac{\sqrt{\sqrt{5} + 2} + \sqrt{\sqrt{5} - 2}}{\sqrt{\sqrt{5} + 1}} \right)^2 \\ &=& \frac{(\sqrt{5} + 2) + 2 \sqrt{(\sqrt{5} + 2)(\sqrt{5} - 2)} + (\sqrt{5} - 2)}{\sqrt{5} + 1)} \\ &=& \frac{2 \sqrt{5} + 2 \sqrt{5-4}}{\sqrt{5} + 1} \\ &=& \frac{2 \sqrt{5} + 2}{\sqrt{5} + 1} = 2. \end{aligned}

So k=2k = \sqrt{2}. The second term, mm does not require squaring:

m=322=222+1=(2)222+1=(21)2=21.m = \sqrt{3 - 2 \sqrt{2}} = \sqrt{2 - 2 \sqrt{2} + 1} = \sqrt{(\sqrt{2})^2 - 2 \sqrt{2} + 1} = \sqrt{(\sqrt{2} - 1)^2} = \sqrt{2} - 1.

Thus N=km=1N = k - m = 1. Surprise!