(a) Expand and simplify in your head:
(i) \((\sqrt{2}+1)^{2}\)
(ii) \((\sqrt{2}-1)^{2}\)
(iii) \((1+\sqrt{2})^{3}\)
(b) Simplify:
(i) \(\sqrt{10+4 \sqrt{6}}\)
(ii) \(\sqrt{5+2 \sqrt{6}}\)
(iii) \(\sqrt{\frac{3+\sqrt{5}}{2}}\)
(iv) \(\sqrt{10-2 \sqrt{5}}\)
The expressions which occur in exercises to develop fluency in working with
surds often appear arbitrary. But they may not be. The arithmetic of surds
arises naturally: for example, some of the expressions in the previous problem
have already featured in Problem \(\mathbf{3}(\mathrm{c}) .\) In particular,
surds will feature whenever Pythagoras' Theorem is used to calculate lengths
in geometry, or when a proportion arising from similar triangles requires us
to solve a quadratic equation. So surd arithmetic is important. For example:
\- A regular octagon with side length 1 can be surrounded by a square of side
\(\sqrt{2}+1\) (which is also the diameter of its incircle); so the area of the
regular octagon equals \((\sqrt{2}+1)^{2}-1\) (the square minus the four
corners).
\- \(\sqrt{2}-1\) features repeatedly in the attempt to apply the Euclidean
algorithm, or anthyphairesis, to express \(\sqrt{2}\) as a "continued fraction".
-\(\sqrt{10-2 \sqrt{5}}\) may look like an arbitrary, uninteresting repeated surd, but is in fact very interesting, and has already featured as \(4 \sin 36^{\circ}\) in Problem \(\mathbf{3}(\mathrm{c})\)
\- One of the simplest ruler and compasses constructions for a regular
pentagon \(A B C D E\) (see Problem 185) starts with a circle of radius 2 ,
centre \(O,\) and a point \(A\) on the circle, and in three steps constructs the
next point \(B\) on the circle, where \(\underline{A B}\) is an edge of the
inscribed regular pentagon, and
$$
\underline{A B}=\sqrt{10-2 \sqrt{5}}
$$