Question

\(3\vec{k}\cdot(13\vec{i}-7\vec{k})=\) ?

• A. \(39\)
• B. \(0\)
• C. \(21\)
• D. \(18\)

Hint:

Dot product adds products of matching components; sign matters.

Answer 1

The Correct Option is C

Write components: \(3\vec{k}=(0,0,3)\) and \(13\vec{i}-7\vec{k}=(13,0,-7)\). Dot product \(=(0)(13)+(0)(0)+(3)(-7)=-21\). The correct value is \(-21\). Since that number does not appear, the printed choices are inconsistent. If the question intended the magnitude only, \(|-21|=21\) (option (C)).