1.

The vector \[\vec{P}=a\hat{i}+a\hat{j}+3\hat{k}\] and \[\vec{Q}=a\hat{i}-2\hat{j}-\hat{k}\] are perpendicular to each other. The positive value of a is

A. 3
B. 4
C. 9
D. 13
Answer» B. 4


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