Tìm hiệu sau theo cách đặt tính trừ:

Lời giải:

Đặt phép tính ta được:$\begin{array}{l}-\underset{\_}{\begin{array}{c}\begin{array}{l}-x^3\\ \end{array}\end{array}\begin{array}{c}\begin{array}{l}\\ \end{array}\end{array}\begin{array}{c}\begin{array}{l}\\ \end{array}\end{array}\begin{array}{c}\begin{array}{l}-\\ \end{array}\end{array}\begin{array}{c}\begin{array}{l}5\text{x}\\ 3\text{x}\end{array}\end{array}\begin{array}{c}\begin{array}{l}+\\ +\end{array}\end{array}\begin{array}{c}\begin{array}{l}2\\ 8\end{array}\end{array}}\\ -x^3-8\text{x}-6\end{array}$

Vậy (-x³ - 5x + 2) - (3x + 8) = -x³ - 8x - 6.