Đáp án và lời giải
Đáp án:A
Lời giải:Ta có: $\begin{gathered} 11 \times 21 = \left( {10 + 1} \right) \times 21 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 10 \times 21 + 21 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 210 + 21 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 231 \ \\ \end{gathered} $ $\begin{gathered} 11 \times 85 = \left( {10 + 1} \right) \times 85 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\, = 10 \times 85 + 85 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\, = 850 + 85 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\, = 935. \ \\ \end{gathered} $ $\begin{gathered} 11 \times 76 = \left( {10 + 1} \right) \times 76 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 10 \times 76 + 76 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 760 + 76 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 836. \ \\ \end{gathered} $ $\begin{gathered} 22 \times 11 = 22 \times \left( {10 + 1} \right) \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 22 \times 10 + 22 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 220 + 22 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 242. \ \\ \end{gathered} $ Vậy khẳng định đúng là: $11 \times 21 = 231$.
