a: \(\Leftrightarrow n+1+4⋮n+1\)

\(\Leftrightarrow n+1\in\left\{1;-1;2;-2;4;-4\right\}\)

hay \(n\in\left\{0;-2;1;-3;3;-5\right\}\)

b: \(\Leftrightarrow n+2-9⋮n+2\)

\(\Leftrightarrow n+2\in\left\{1;-1;3;-3;9;-9\right\}\)

hay \(n\in\left\{-1;-3;1;-5;7;-11\right\}\)

c: \(\Leftrightarrow2n-2+8⋮n-1\)

\(\Leftrightarrow n-1\in\left\{1;-1;2;-2;4;-4;8;-8\right\}\)

hay \(n\in\left\{2;0;3;-1;5;-3;9;-7\right\}\)

\(n+3⋮2n+1\)

\(\Leftrightarrow2\left(n+3\right)⋮2n+1\)

\(\Leftrightarrow2n+6⋮2n+3\)

\(\Leftrightarrow\left(2n+3\right)+3⋮2n+3\)

Vì \(2n+3⋮2n+3\)

\(\Rightarrow6⋮2n+3\)

\(\Rightarrow2n+3\inƯ\left(6\right)=\){ 

1 / ab = 42 = 1.42 = 2.21 = 3.14 = 6.7

Vậy  \(\left(a,b\right)\in\left\{\left(1,42\right);\left(2,21\right);\left(3,14\right);\left(6,7\right)\right\}\)

2/ ab = 30 = 1.30 = 2.15 = 3.10 = 5.6 

Vậy \(\left(a,b\right)\in\left\{\left(1,30\right);\left(2,15\right);\left(3,10\right);\left(5,6\right)\right\}\)

3 / ab = 36 = 1.36 = 2.18 = 3.12 = 4.9 

Vậy : \(\left(a,b\right)\in\left\{\left(1,36\right);\left(2,18\right);\left(3,12\right);\left(4,9\right)\right\}\)

4 / Đề chỉ cho b = 40 chứ không có a

1) ab= 42=6.7=(-6).(-7)=1.42=(-1).(-42)=2.21=(-2).(-21)

2) ab= 30= 5.6 =(-5).(-6)=1.30=(-1).(-30)=2.15=(-2).(-15)

3) ab=36 =2.18=(-2).(-18)=3.12=(-3).(-12)=1.36= (-1).(-36)

Ta có: \(\frac{1}{a}\)+\(\frac{1}{b}\)=\(\frac{2}{143}\)\(\Rightarrow\)\(\frac{a+b}{ab}\)=\(\frac{2}{143}\)\(\Rightarrow\)143(a+b)=2ab (1)

Mặt khác: a-b=2\(\Rightarrow\)a=2+b (2)

Thay (2) vào (1) ta có:

143(2+b+b)=2(2+b)b\(\Leftrightarrow\)286+286b=b(4+2b)=286+286b=4b+2bb\(\Leftrightarrow\)2bb+4b-286b-286=0\(\Leftrightarrow\)2bb-282b-286=0

ta có a=11;b=13