(a) Show that a=(2+\sqrt{2})(3+\sqrt{3}) is not a square in F=\mathbb{Q}(\sqrt{2},\sqrt{3}).
(b) Conclude from (a) that [E~:~\mathbb{Q}]=8. Prove that the roots of the minimal polynomial over \mathbb{Q} for α are the 8 elements \pm \sqrt{(2 \pm \sqrt{2})(3 \pm \sqrt{3})}
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(f) Conclude \text{Gal}(E/\mathbb{Q})≅Q_8.
Proof: (a) We observe that if a=c^2 then aφa=c^2φc^2=(cφc)^2 where φ is the automorphism of F fixing \sqrt{2} and negating \sqrt{3}. Since cφc is actually the image of c under N_{F/\mathbb{Q}(\sqrt{2})} we have cφc=\sqrt{aφa}=\sqrt{6(3+\sqrt{3})^2}=\pm (3\sqrt{2}+3\sqrt{6})∈\mathbb{Q}(\sqrt{2}) and now \sqrt{6}∈\mathbb{Q}(\sqrt{2}), a contradiction.
(b-f) (We take a slightly different path from the authors, especially for parts c and d) We have shown [\mathbb{Q}(\sqrt{2},\sqrt{3},α)~:~\mathbb{Q}]=8. Since \dfrac{α^2}{2+\sqrt{2}}-3=\sqrt{3}, we have \mathbb{Q}(\sqrt{2},\sqrt{3},α)=\mathbb{Q}(\sqrt{2},α)=E(\sqrt{2}). Assume [E(\sqrt{2})~:~E]=2; then E(\sqrt{2}) is Galois of degree 2 over E (splitting x^2-2), and the map φ:\sqrt{2}↦-\sqrt{2} is an automorphism of E(\sqrt{2}) fixing E. But φ is in particular an isomorphism of F allowing us to observe φ(α^2)=φ((2+\sqrt{2})(3+\sqrt{3}))=(2-\sqrt{2})(3+\sqrt{3}) \neq α^2, a contradiction. So E=E(\sqrt{2}) and [E~:~\mathbb{Q}]=8.
Now as we saw above, E is the splitting field for x^2-α^2 over F. Therefore every automorphism of F extends to an automorphism of E. This is an order 4 subgroup of \text{Aut}(E/\mathbb{Q}). Since E/F is Galois we also have the automorphism ψ:α↦-α fixing F. Letting n=|\text{Aut}(E/\mathbb{Q})| by Lagrange we see 4~|~n, by Galois we see n≤8, and by counting we see n > 4, so that n=8 and E/\mathbb{Q} is Galois.
We see that the 8 elements mentioned above are distinct by observing squares, and since φ(x^2)=φ(x)^2 for general automorphisms we have φ(x)=\pm \sqrt{φ(x^2)}. Letting H=\text{Aut}(E/F) we see 1,ψ form a set of right coset representatives for H in \text{Aut}(E/\mathbb{Q}). By letting λ∈H fix \sqrt{3} and negate \sqrt{2}, for example, we see λψ(α^2)=(2-\sqrt{2})(3+\sqrt{3}) and thus λψ(α)=\pm \sqrt{(2-\sqrt{2})(3+\sqrt{3})}. Similarly, we can see any automorphism maps α to one of the 8 forms above, and thus must map to all of them, and these are the 8 distinct roots of the minimal polynomial for α over \mathbb{Q}.
Let σ map α to β=\sqrt{(2-\sqrt{2})(3+\sqrt{3})}. We see σ(α^2)=β^2 so that σ(\sqrt{2})=-\sqrt{2} and σ(\sqrt{3})=\sqrt{3}, and together with αβ=\sqrt{2}(3+\sqrt{3}) we have σ(αβ)=-αβ and thus σ(β)=-α. Now σ can be seen to be of order 4 and together with \tau mapping α to γ=\sqrt{(2+\sqrt{2})(3-\sqrt{3})} we similarly find the relations σ^4=\tau^4=1, σ^2=\tau^2, and σ\tau = \tau σ^3 (keeping in mind β=\dfrac{\sqrt{2}(3+\sqrt{3})}{α}), so that \text{Gal}(E/\mathbb{Q})≅Q_8.~\square