<s>[INST] Determining if a subset of $\mathbb{C}$ is a subfield. I am trying to deduce whether or not 
$$K=\{a+b\sqrt{2}|a,b\in\mathbb{Q}\text{ and }ab<\sqrt{2}\}$$
is a subfield of $\mathbb{C}$.  I feel like I'm getting stuck.  If I subtract two numbers $a+b\sqrt{2}$ and $c+d\sqrt{2}$, then we have to show that 
$$(a-c)(b-d)=ab+cd-(ad+bc)<2\sqrt{2}-(ad+bc)<\sqrt{2}$$
Thus I have $ad+bc>\sqrt{2}$ and I'm not sure how to procede here.  It I look at the multiplicative inverse of $a+b\sqrt{2}$,
$$(a+b\sqrt{2})^{-1}=\frac{a}{a^2-2b^2}+\frac{-b}{a^2-2b^2}\sqrt{2}$$
and so the conditions must hold for 
$$\frac{-ab}{(a^2-2b^2)^2}<\frac{-\sqrt{2}}{(a^2-2b^2)^2}<\sqrt{2}$$
$$\frac{1}{(a^2-2b^2)^2}>-1$$
Which is true for all $a,b$.  I'm stuck....
 [/INST] The key here is observing that, for $a,b,c,d\in\mathbb{Q}$,
$$
a+b\sqrt{2}=c+d\sqrt{2}
\quad\text{if and only if}\quad
a=c\text{ and }b=d
$$
which is easy, but should be noted before going on. One direction is obvious; for the reverse, if $b\ne d$, then
$$
\sqrt{2}=\frac{a-c}{d-b}
$$
which is a contradiction. Therefore $b=d$ and consequantly also $a=c$.
Now $2=2+0\sqrt{2}$ and $2\sqrt{2}=0+2\sqrt{2}$ both belong to $K$. However $2+2\sqrt{2}$ doesn't satisfy $2\cdot2<\sqrt{2}$ and there cannot be other rational numbers $a,b$ with $a+b\sqrt{2}=2+\sqrt{2}$. Thus $K$ is not closed under addition.
</s>