Câu hỏi 21

* Ta có :

\(\begin{array}{l}\overrightarrow {AB}  = \left( {1; - 2;1} \right);\,\,2\overrightarrow {BC}  = \left( {2; - 6;10} \right) \Rightarrow \overrightarrow {AB}  + 2\overrightarrow {BC}  = \left( {3; - 8;11} \right)\\*\,\,\left| {\overrightarrow {AB}  + 2\overrightarrow {BC} } \right| = \sqrt {{3^2} + {8^2} + {{11}^2}}  = \sqrt {194} \end{array}\)

Chọn D.