Question

OA and BO are two vectors of magnitudes 5 and 6 respectively. If $\angle B O A=60^{\circ}$, then $0 A \cdot O B$ is equal to

• A. $15$
• B. $0$
• C. $15 \sqrt {3}$
• D. $-15$

Answer 1

Answer: (A)

We have magnitudes of $OA$ and $OB$ are 5 and 6 respectively
And $\angle BOA=60^{\circ}$
Now, $ O A \cdot O B =| O A | \cdot O B \mid \cos \theta$
$\Rightarrow 0 A \cdot O B =5 \cdot 6 \cdot \cos 60^{\circ}$
$\Rightarrow O A \cdot O B =30 \times \frac{1}{2}$
$\Rightarrow OA \cdot O B =15$