$\frac{(2x+1)^2}{5}$ - $\frac{(x-1)^2}{3}$= $\frac{7x^2-14x-5}{15}$
$\Leftrightarrow$ $\frac{(2x+1)^2.3}{15}$ - $\frac{(x-1)^2.5}{15}$= $\frac{7x^2-14x-5}{15}$
$\Leftrightarrow$ $12x^2$ + 12x +3 - ( $5x^2$ -10x+5) = $7x^2$ -14x-5
$\Leftrightarrow$ $12x^2$ + 12x +3 - $5x^2$ +10x-5= $7x^2$ -14x-5
$\Leftrightarrow$ $7x^2$ +22x-2= $7x^2$ -14x-5
$\Leftrightarrow$ $7x^2$ - $7x^2$ +22x+14x= -5+2
$\Leftrightarrow$ 36x =-3
$\Leftrightarrow$ x= $\frac{-1}{12}$
Vậy S={$\frac{-1}{12}$}
