Question

Liệt kê các tập hợp sau:

B = {n ∈ N | n (2n + 1) ≤ 78}

C = {4k - 1 | k ∈ Z, 0 ≤ k < 3}

Gấp ạ! Nhanh please

Answer 1

\(B=\left\{0;1;2;3;4;5;6\right\}\\ C=\left\{-1;3;7\right\}\)

Answer 2

1) \(n\left(2n+1\right)\le78\)

\(\Leftrightarrow2n^2+n-78\le0\)

\(\Leftrightarrow\left(n-6\right)\left(2n+13\right)\le0\)

\(\Leftrightarrow\left\{{}\begin{matrix}n-6\le0\\2n+13\ge0\end{matrix}\right.\)\(\Leftrightarrow\dfrac{-13}{2}\le n\le6\)

Vậy \(B=\left\{0;1;2;3;4;5;6\right\}\)

2) \(0\le k< 3,k\in Z\Rightarrow k\in\left\{0;1;2\right\}\)

\(\Rightarrow4k-1\in\left\{-1;3;7\right\}\)

Vậy \(C=\left\{-1;3;7\right\}\)