<s>[INST] Prove the following set has a multiplicative inverse Let $F = \{x + y\sqrt{7} : x, y \in Q\}$ with the usual addition and multiplication operations. Show that $F$ has a multiplicative inverse.
So far, I have
$$\left(x_1 + y_1\sqrt{7}\right) \left(\frac{1}{x_1} + \left(\frac{1}{y_1}\right)\sqrt{7}\right)$$
But that led me to $8+2\sqrt{7}(\frac{1}{x_1} + \frac{1}{y_1}) = 1$...I don't know what goes from there.
 [/INST] Suppose $(a+b\sqrt{7})(x+y \sqrt{7}) = 1$ (and $(x,y) \neq (0,0)$). This gives $(ax+7 by) + (xb+ay)\sqrt{7} = 1$, from which we get two equations: $ax+7 by = 1$, $xb+ay = 0$. Now solve for $a,b$ to get
$a = \frac{x}{x^2-7 y^2}$, $b=-\frac{y}{x^2-7 y^2}$.
To see that $x^2 \neq 7 y^2$, note that $\sqrt{7}$ is irrational.
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