a, $( x-12).( 8+x)= 0$
⇔ \(\left[ \begin{array}{l}x-12= 0\\8+x= 0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=12\\x=-8\end{array} \right.\)
b, 2n-1⋮ n+1
⇔ 2.( n+1)-3⋮ n+1
⇒ 3⋮ n+1
⇒ n+1∈ Ư( 3)= { -3; -1; 1; 3}
⇔ x∈ { -4; -2; 0; 2}
c, A= | x-2|+| y+5|-15
Vì | x-2|≥ 0 ∀x
| y+5|≥ 0 ∀y
⇒ A≥ -15
Dấu = xảy ra khi x= 2; y= -5