Question

For what value of λ are the vectors \[\vec{a} \text{ and  }\vec{b}\] perpendicular to each other if \[\vec{a} = \lambda \hat{i} + 2 \hat{j} + \hat{k} \text{ and } \vec{b} = 4\hat{i} - 9 \hat{j} + 2\hat{k}\] 

Answer 1

\[ \text{If the vectors }\vec{a} \text{ and } \vec{b} \text{ are perpendicular to each other, then }\]

\[ \vec{a} . \vec{b} = 0\]                                           

\[ \Rightarrow \left( \lambda \hat{i} + 2\hat {j}+ \hat{k} \right) . \left( 4 \hat{i} - 9 \hat{j} + 2 \hat{k} \right) = 0\]

\[ \Rightarrow 4\lambda - 18 + 2 = 0\]

\[ \Rightarrow 4\lambda - 16 = 0\]

\[ \Rightarrow 4\lambda = 16\]

\[ \Rightarrow \lambda = 4\]