[คนไม่เก่งที่ชอบคณิต]

3)
$2a + \frac{1}{b} = 1$ --(1)
$b - \frac{1}{c} = 1$ --(2)

$bc\times(1)$ $2abc + c = bc$

$c\times(2)$ $bc - 1 = c$

$abc = \frac{1}{2}$

$abc^{2} = \frac{1}{4}$