Prove the inequality $\sum_{1\le iPlease demonstrate this is true
This is the exercise: 
$$\sqrt{a_1a_2}+\sqrt{a_1a_3}+\ldots+\sqrt{a_1a_n}+\sqrt{a_2a_3}+\ldots+\sqrt{a_{n-1}a_n}<\frac{n-1}2(a_1+a_2+a_3+\ldots+a_n).$$
I tried to solve it, but I couldn't do anything right.
This is my idea:
$\sqrt{a_1a_2}<\frac{a_1+a_2}2$ -because geometric mean < arithmetic mean;
$\sqrt{a_1a_3}<\frac{a_1+a_3}2$,
$\dots$,
$\sqrt{a_{n-1}a_n}<\frac{a_{n-1}+a_n}2$
Please give me the answer!!
$a_1,a_2,a_3,\ldots,a_n$ are real, positive nummbers

Use the AM-GM inequality, which states that:
$$\sqrt{a_ia_j}\le\frac{a_i+a_j}{2}$$
Then we have the following:
$$\sum_{i< j}\sqrt{a_ia_j}\le\sum_{i< j}\frac{a_i+a_j}{2}=\frac{1}{2}\sum_{i< j}a_i+a_j\\=\frac{1}{2}[(a_1+a_2)+(a_1+a_3)+\ldots+(a_1+a_n)+(a_2+a_3)+\ldots+(a_{n-1}+a_n)]$$
Now notice that in the sum on the right hand side, each $a_i$ appears $n-1$ times, so we have:
$$\sum_{i< j}\sqrt{a_ia_j}\le\frac{n-1}{2}(a_1+\ldots+a_n)$$
