A) \[11\sqrt{5}\]
B) \[11\sqrt{3}\]
C) \[11\sqrt{7}\]
D) \[11\sqrt{2}\]
Correct Answer: A
Solution :
\[(\mathbf{a}\times \mathbf{b})=\left| \,\begin{matrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ 2 & 4 & -5 \\ 1 & 2 & 3 \\ \end{matrix}\, \right|=22\mathbf{i}-11\mathbf{j}+0\mathbf{k}\] \[(\mathbf{a}\times \mathbf{b})=\sqrt{{{(22)}^{2}}+{{(11)}^{2}}}\]\[=11\sqrt{{{2}^{2}}+1}=11\sqrt{5}\].
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