5n + 7 \(⋮\) 3n + 2
=> 15n + 21 \(⋮\) 3n + 2
=> 15n + 10 + 11 \(⋮\) 3n + 2
=> 11 \(⋮\) 3n + 2
=> 3n + 2 \(\in\) Ư(11)
=> 3n + 2 = { 1 ; -1 ; 11 ; -11}
Vì n \(\in\) Z
=> n = { -1 ; 3 }
Điều kiện \(3n+2\ne0\)
Ta có:
\(5n+7⋮3n+2\Rightarrow3.\left(5n+7\right)⋮3n+2\) (1)
\(5.\left(3n+2\right)⋮3n+2\) (2)
Từ (1) và (2) \(\Rightarrow3.\left(5n+7\right)-5.\left(3n+2\right)⋮3n+2\Leftrightarrow15n+21-15n-10⋮3n+2\)
\(\Rightarrow11⋮3n+2\) \(\Rightarrow3n+2\inƯ\left(11\right)=\left\{1;-1;11;-11\right\}\)
+) Với 3n+2=1 => 3n=-1 => \(n=-\dfrac{1}{3}\) (ko thỏa mãn)
+) Với 3n+2=-1 => 3n=-3 => n=-1 (thỏa mãn)
+) Với 3n+2=11 => 3n=9 => n=3 (thỏa mãn)
+) Với 3n+2=-11 => 3n=-13 \(\Rightarrow n=-\dfrac{13}{3}\) (không thỏa mãn)
Vậy n=-1 hoặc n=3
Ta có:
\(\dfrac{5n+7}{3n+2}=\dfrac{15n+21}{3n+2}=\dfrac{15n+10+11}{3n+2}\)
\(=\dfrac{15n+10}{3n+2}+\dfrac{11}{3n+2}=3+\dfrac{11}{3n+2}\)
\(\Rightarrow3n+2\inƯ\left(11\right)\)
\(\Rightarrow3n+2\in\left\{-11;-1;1;11\right\}\)
\(\Rightarrow3n\in\left\{-13;-3;-1;9\right\}\)
\(\Rightarrow n\in\left\{\dfrac{-13}{3};-1;\dfrac{-1}{3};3\right\}\)
mà \(n\in Z\) nên \(n\in\left\{-1;3\right\}\)
Vậy \(n\in\left\{-1;3\right\}\)
Chúc bạn học tốt!!!
\(5n+7⋮3n+2\)
\(3\left(5n+7\right)⋮3n+2\)
\(15n+21⋮3n+2\)
\(15n+10+11⋮3n+2\)
\(5\left(3n+2\right)+11⋮3n+2\)
\(\Leftrightarrow11⋮3n+2\)
\(\Rightarrow3n+2\inƯ\left(11\right)\)
\(Ư\left(11\right)=\left\{\pm1;\pm11\right\}\)
\(n=\left\{-1;3\right\}\)