[กขฃคฅฆง]

$\dfrac{1}{a} +\dfrac{1}{b} +\dfrac{1}{c} +\dfrac{1}{d} < \dfrac{1}{a} +\dfrac{1}{a} +\dfrac{1}{a} +\dfrac{1}{a} $

$1 < \dfrac{4}{a} $

$a<4$