Question

For what value of λ are the vectors \[\vec{a} \text{ and } \vec{b}\] perpendicular to each other if  

\[\vec{a} = \lambda \hat{i} + 3 \hat{j} + 2 \hat{k}\text { and } \vec{b} = \hat{i} - \hat{j} + 3 \hat{k}\]

Answer 1

\[\text{ If the vectors } \vec{a} \text{ and } \vec{b} \text{ are perpendicular to each other, then }\]
\[ \vec{a} . \vec{b} = 0\]
\[ \Rightarrow \left( \lambda \hat{i} + 3 \hat{j} + 2 \hat{k}\right) . \left( \hat{i} - 1\hat{j} + 3\hat{k} \right) = 0\]
\[ \Rightarrow \lambda - 3 + 6 = 0\]
\[ \Rightarrow \lambda + 3 = 0\]
\[ \Rightarrow \lambda = - 3\]