Để phân số \(M=\frac{3N+4}{N-1}\inℤ\)thì \(3N+4⋮N+1\)
Ta có :
\(3N+4=N+N+N+4\)
\(=\left(N+1\right)+\left(N+1\right)+\left(N+1\right)+4-3\)
\(=3\left(N+1\right)+1\)
Vì \(N+1⋮N+1\)nên \(3\left(N+1\right)⋮\left(N+1\right)\)
Vì \(3\left(N+1\right)⋮N+1\)nên để \(3\left(N+1\right)+1⋮N+1\)thì \(1⋮N+1\)
\(\Rightarrow N+1\in\left\{1;-1\right\}\)
\(\Rightarrow N\in\left\{0;-2\right\}\)
Vậy \(N\in\left\{0;-2\right\}\)
Ta có :
\(M=\frac{3n+4}{n-1}=\frac{3n-3+7}{n-1}=\frac{3n-3}{n-1}+\frac{7}{n-1}=\frac{3\left(n-1\right)}{n-1}+\frac{7}{n-1}=3+\frac{7}{n-1}\)
Để \(M\) là số nguyên thì \(7⋮\left(n-1\right)\) \(\Rightarrow\) \(\left(n-1\right)\inƯ\left(7\right)\)
Mà \(Ư\left(7\right)=\left\{1;-1;7;-7\right\}\)
Suy ra :
| \(n-1\) | \(1\) | \(-1\) | \(7\) | \(-7\) |
| \(n\) | \(2\) | \(0\) | \(8\) | \(-6\) |
Vậy \(n\in\left\{-6;0;2;8\right\}\)
