a
બે પરસ્પર લંબ સદીશોનો ડોટ ગુણાકાર શૂન્ય હોય છે . 

\(\begin{array}{l}
\mathop A\limits^ \to  .\,\,\mathop B\limits^ \to  \, = \,\,0 \\
\,\therefore \,\,\left( {4\hat i\, + \,\,n\hat j\,\, - \,\,2\hat k} \right).\,\,\left( {2\hat i\,\, + \;\,3\hat j\,\, + \;\,\hat k} \right) = \,\,0\,\, \\ \Rightarrow \,\,\left( {4\,\, \times \,\,2} \right)\,\, + \,\,\left( {n\,\, \times \,\,3} \right)\,\, + \;\,\left( { - 2\,\, \times \,\,1} \right)\,\, = \,\,0\,\,\\ \Rightarrow \,\,3n\,\, =  - 6\,\,\\ \Rightarrow \,\,n\,\, = \,\, - 2
\end{array}\)