Question

If \[\vec{a} = 3 \hat { i } + 4 \hat { j } \text{ and }  \vec{b} = \hat { i  } + \hat{ j }  + \hat{ k } ,\]  find the value of \[\left| \vec{a} \times \vec{b} \right| .\]

 

Answer 1

\[\text{ Given } : \]
\[ \vec{a} = 3 \hat{ i  } + 4 \hat{ j }  + 0 \hat{ k }  \]
\[ \vec{b} = \hat{ i }  + \hat{ j }  + \hat{ k }  \]
\[ \therefore \vec{a} \times \vec{b} = \begin{vmatrix}\hat{ i } & \hat{ j }  & \hat{ k }  \\ 3 & 4 & 0 \\ 1 & 1 & 1\end{vmatrix}\]
\[ = \left( 4 - 0 \right) \hat{ i } - \left( 3 - 0 \right) \hat{ j } + \left( 3 - 4 \right) \hat{ k } \]
\[ = 4 \hat{ i }- 3 \hat{ j } - \hat{ k } \]
\[ \Rightarrow \left| \vec{a} \times \vec{b} \right| = \sqrt{16 + 9 + 1}\]
\[ = \sqrt{26}\]