<s>[INST] How to show this conjecture：$\frac{1}{\sqrt{a_{1}}}+\frac{1}{\sqrt{a_{2}}}+\cdots+\frac{1}{\sqrt{a_{n}}}\ge\frac{2n}{\sqrt{a_{1}}+\sqrt{a_{n}}}$ I have prove this inequality 
$$\dfrac{1}{\sqrt{1}}+\dfrac{1}{\sqrt{2}}+\cdots+\dfrac{1}{\sqrt{n}}>2\sqrt{n+1}-2$$
because
$$\sum_{k=1}^{n}\dfrac{1}{\sqrt{k}}>\sum_{k=1}^{n}\dfrac{2}{\sqrt{k}+\sqrt{k+1}}=2\sum_{k=1}^{n}(\sqrt{k+1}-\sqrt{k})=2\sqrt{n+1}-2$$
so
$$\sum_{k=1}^{n}\dfrac{1}{\sqrt{k}}>2\sqrt{n+1}-2$$
Now I conjecture：

if postive arithmetic progression $\{a_{n}\}$ ,and the common difference of successive members is $d>0$.have
  $$\dfrac{1}{\sqrt{a_{1}}}+\dfrac{1}{\sqrt{a_{2}}}+\cdots+\dfrac{1}{\sqrt{a_{n}}}\ge\dfrac{2n}{\sqrt{a_{1}}+\sqrt{a_{n}}}$$

 [/INST] Using the AM-HM inequality states: if $x_1\leq\ldots\leq x_n$ are nonnegative numbers, then
\begin{align}
\frac{x_1+\ldots +x_n}{n} \geq \frac{n}{\frac{1}{x_1}+\ldots+\frac{1}{x_n}}
\end{align}
which implies
\begin{align}
\frac{1}{x_1} + \ldots + \frac{1}{x_n} \geq \frac{n^2}{x_1+ \ldots + x_n} \geq \frac{n}{x_1 + x_n}.
\end{align}
However, we might be able to do better.
Special Case: Let us consider arithmetic progression of the form $a_k = dk$ for $k = 1, \dots, n$, then the desired inequality becomes
\begin{align}
\sum^n_{k=1} \frac{1}{\sqrt{k}} \geq \frac{2n}{1+\sqrt{n}}  \ \  \ (*)
\end{align}
To establish $(\ast)$, we will use Euler Maclaurin Series
\begin{align}
\sum^n_{k=1} \frac{1}{\sqrt{k}} = 1 + \int^n_1\frac{dx}{\sqrt{x}} + \frac{1}{2}\left(\frac{1-\sqrt{n}}{\sqrt{n}} \right)+\frac{1}{24}\left( 1-n^{-3/2}\right)+R
\end{align}
where 
\begin{align}
|R| \leq \frac{3\zeta(2)}{2(2\pi)^2}\int^n_1 \frac{dx}{x^{5/2}} = \frac{1}{24}(1-n^{-3/2}).
\end{align}
Then it follows
\begin{align}
\sum^n_{k=1} \frac{1}{\sqrt{k}}-\frac{2n}{1+\sqrt{n}} =&\ 1+ \frac{2(n-1)}{1+\sqrt{n}}+\frac{1}{2}\left(\frac{1-\sqrt{n}}{\sqrt{n}} \right)-\frac{2n}{1+\sqrt{n}}+\text{ something positive}\\
\geq&\  \frac{\sqrt{n}-1}{\sqrt{n}+1}+\frac{1}{2}\left(\frac{1-\sqrt{n}}{\sqrt{n}} \right) = \frac{1}{2}\frac{(\sqrt{n}-1)^2}{\sqrt{n}(\sqrt{n}+1)}.\\
\end{align}
Thus, it follows
\begin{align}
\sum^n_{k=1} \frac{1}{\sqrt{k}} \geq \frac{2n}{1+\sqrt{n}}.
\end{align}
Comment: Using the same idea, we might be able to work with arithmetic progression of the form $a_k = d(m+k)$ for $k=1, \ldots, n$. However, I have no clue how to get the most general arithmetic progression $a_k = d(m+k)+\ell$. 
</s>