Question

If the moduli of a and b are equal and angle between them is $120 ^ { \circ }$ and $\mathbf { a } \cdot \mathbf { b } = - 8$ then | a | is equal to

• A. – 5
• B. – 4
• C. 4
• D. 5

Answer 1

Answer: (C)

4

Answer 2

$\mathbf { a } \cdot \mathbf { b } = a a \cos 120 ^ { \circ }$,

$\Rightarrow - 8 = - \frac { a ^ { 2 } } { 2 } \Rightarrow a = 4$

(Negative sign does not occur in moduli).