Đáp án:
Giải thích các bước giải:
$y'=3 \ge 0\\ \text{đồng biến trên }\mathbb{R}\\ \text{ta có BBT :}\\\begin{array}{|c|cc|}\hline \text{$x$}&\text{$-\infty$}&\text{}&\text{}&\text{}&\text{}&\text{}&\text{$+\infty$}\\\hline \text{$y'$}&\text{}&\text{}&\text{}&+\text{}&\text{}&\text{}&\\\hline \text{$y$}&\text{}&\text{}&\text{}&\text{}&\text{}&\text{}\\&\text{}&\text{$$}&\text{}&\text{}↗&\text{}&\text{}\\&\text{$$}&\text{}&\text{}&\text{}&\text{}&\text{}&\text{}\\\hline \end{array}\\\mathscr{X_{\textit{in hay nhất}}}$
