cho biểu thức sau: `B=((a-a^2-3)/(a^2-1)-1/(a-1)):((a+1)/(a-1)-(a-1)/(a+1)-(8a)/(a-1))` hãy rút gọn B?

$B=\left(\dfrac{a-a^2-3}{a^2-1}-\dfrac{1}{a-1}\right):\left(\dfrac{a+1}{a-1}-\dfrac{a-1}{a+1}-\dfrac{8a}{a-1}\right)$ `;(a ≠ 0 ; a ≠ ± 1; a ≠ − 1/ 2 )`

$=\left(\dfrac{-a^2-4}{\left(a+1\right)\left(a-1\right)}\right)\::\:\left(\dfrac{-8a^2-4a}{\left(a-1\right)\left(a+1\right)}\right)$

$=\dfrac{\left(-a^2-4\right)\left(a-1\right)\left(a+1\right)}{\left(a+1\right)\left(a-1\right)\left(-8a^2-4a\right)}$

$=\dfrac{\left(-a^2-4\right)\left(a+1\right)}{\left(a+1\right)\left(-8a^2-4a\right)}$

$B=\dfrac{-a^2-4}{-8a^2-4a}$ với `a ≠ 0 ; a ≠ ± 1; a ≠ − 1 /2`