a: =>12n+18 chia hết cho 4n-1
=>12n-3+21 chia hết cho 4n-1
=>\(4n-21\in\left\{1;-1;3;-3;7;-7;21;-21\right\}\)
hay \(n\in\left\{\dfrac{11}{2};5;6;\dfrac{9}{2};7;\dfrac{7}{2};\dfrac{21}{2};0\right\}\)
=>\(n\in\left\{\dfrac{11}{2};5;6;\dfrac{9}{2};7;\dfrac{7}{2};\dfrac{21}{2}\right\}\)
b: =>-2n-2+17 chia hết cho n+1
=>\(n+1\in\left\{1;-1;17;-17\right\}\)
hay \(n\in\left\{0;-2;18\right\}\)