MCQ
If $A = [1\,2\,3],B = \left[ \begin{array}{l}2\\3\\4\end{array} \right]$ and $C = \left[ {\begin{array}{*{20}{c}}1&5\\0&2\end{array}} \right]$, then which of the following is defined
  • A
    $AC$
  • $BA$
  • C
    $(AB)\,{\rm{. }}C$
  • D
    $(AC)\,.\,B$

Answer

Correct option: B.
$BA$
b
(b) $BA = {\left[ \begin{array}{l}2\\3\\4\end{array} \right]_{3 \times 1}}\,{[1\,\,2\,\,3]_{1 \times 3}}$ $ = {\left[ {\begin{array}{*{20}{c}}2&4&6\\3&6&9\\4&8&{12}\end{array}} \right]_{3 \times 3}}$

$AB = {[1\,2\,3]_{1 \times 3}}{\left[ \begin{array}{l}2\\3\\4\end{array} \right]_{3 \times 1}} = {[20]_{1 \times 1}}$.

So, $AB$ and $BA$ are defined.

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