KNOW that \[A>
G
>H\] Where \[A\] is arithmetic
MEAN
, \[G\] is geometric mean and \[H\] is harmonic mean, then \[{{S}_{n}}=\frac{1}{4}n\,(n+1)\,(n+2)(n+3)\] \[\Rightarrow \] or \[(a+b)>2\sqrt{
AB
}\].