Đáp án:
Giả sử $a_{1}\leq a_{2}\leq ...\leq a_{n-1}\leq a_{n}$
$a_{1}+a_{2}+...+a_{n-1}> a_{n};a_{n-1}\leq a_{n}\to a_{n}-a_{n-1}\geq 0\Rightarrow a_{1}+a_{2}+...+a_{n-2}> a_{n}-a_{n-1}\geq 0$
$\rightarrow a_{n-2}> 0\rightarrow a_{n}\geq a_{n-1}\geq a_{n-2}> 0$