May cua em bi loi tieng Viet nen em k viet co dau duoc.
For any distinct positive integers $a, b, c$, let $d=\sqrt{a}+\sqrt{b}+\sqrt{c}$. It is sufficient to show that $\sqrt{a} \in \mathbb{Q}\left ( d \right )$.
We have $$\begin{aligned}
\left ( d-\sqrt{a} \right )^2 &= \left ( \sqrt{b}+\sqrt{c} \right )^2\\
\Rightarrow \left ( d^2+a-b-c \right )-2\sqrt{a}d &= 2\sqrt{bc}\\
\left ( d^2+a-b-c \right )^2-4\sqrt{a}d\left ( d^2+a-b-c \right ) +4ad^2 &= 4bc
\end{aligned}$$
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