a, Ta có : \(x+3=\left(x+3\right)^2\)
=> \(\left(x+3\right)-\left(x+3\right)^2=0\)
=> \(\left(x+3\right)\left(1-\left(x+3\right)\right)=0\)
=> \(\left[{}\begin{matrix}x+3=0\\1-\left(x+3\right)=0\end{matrix}\right.\)
=> \(\left[{}\begin{matrix}x=-3\\x=-2\end{matrix}\right.\)
Vậy phương trình có tập nghiệm là \(S=\left\{-2,-3\right\}\)
b, Ta có : \(n^2-4n-15⋮n+2\)
=> \(n^2+4n-8n+4-16-3⋮n+2\)
=> \(\left(n^2+4n+4\right)-\left(8n+16\right)-3⋮n+2\)
=> \(\left(n+2\right)^2-8\left(n+2\right)-3⋮n+2\)
=> \(\left(n+2\right)\left(n-6\right)-3⋮n+2\)
Mà \(\left(n+2\right)\left(n-6\right)⋮n+2\)
=> \(-3⋮n+2\)
=> \(n+2\inƯ_{\left(-3\right)}\)
Mà \(n\in Z\)
=> \(n+2\in\left\{1,-1,3,-3\right\}\)
=> \(n\in\left\{-1,-3,1,-5\right\}\)
Vậy \(n\in\left\{-1,-3,1,-5\right\}\) để n2- 4n - 15 chia hết cho n + 2
