$a) \dfrac{x+1}{15}+\dfrac{x+1}{16}-\dfrac{x+1}{17}-\dfrac{x+1}{18}=0$
$⇔(x+1)\left (\dfrac{1}{15}+\dfrac{1}{16}-\dfrac{1}{17}-\dfrac{1}{18} \right )=0$
$⇔x+1=0$ Vì $\dfrac{1}{15}+\dfrac{1}{16}-\dfrac{1}{17}-\dfrac{1}{18} \neq 0$
$⇔x=-1$
Vậy $x=-1$
$b) \dfrac{x+1}{65}+1+\dfrac{x+3}{63}+1-\dfrac{x+5}{61}-1-\dfrac{x+7}{59}-1=0$
$⇔\dfrac{x+66}{65}+\dfrac{x+66}{63}-\dfrac{x+66}{61}-\dfrac{x+66}{59}=0$
$⇔(x+66)\left (\dfrac{1}{65}+\dfrac{1}{63}-\dfrac{1}{61}-\dfrac{1}{59} \right )=0$
$⇔x+66=0$ Vì $\dfrac{1}{65}+\dfrac{1}{63}-\dfrac{1}{61}-\dfrac{1}{59} \neq 0$
$⇔x=-66$
Vậy $x=-66$
