$a) \frac{x +1}{35} + \frac{x +3}{33} = \frac{x +5}{31} + \frac{x +7}{29}$
$⇔ \frac{x +1}{35} +1 + \frac{x +3}{33} +1 = \frac{x +5}{31} +1 + \frac{x +7}{29} +1$
$⇔ \frac{x +36}{35} + \frac{x +36}{33} - \frac{x +36}{31} - \frac{x +36}{29} = 0$
$⇔ (x +36).(\frac{1}{35} + \frac{3}{33} - \frac{5}{31} - \frac{7}{29}) = 0$
$⇔ x +36 = 0$ $(Vì$ $\frac{1}{35} + \frac{3}{33} - \frac{5}{31} - \frac{7}{29} \neq 0)$
$⇔ x = -36$
$Vậy$ $S =$ {$-36$}
