The angle between two vectors $4\hat{i} + 3\hat{j} + \hat{k}$ and $-3\hat{i} + 2\hat{j} + 6\hat{k}$ is ....... $^o$

Which of the following is not true about vectors $\vec{A}, \vec{B}$ and $\vec{C}$?

If $\overrightarrow{P} \cdot \overrightarrow{Q} = PQ,$ then the angle between $\overrightarrow{P}$ and $\overrightarrow{Q}$ is ....... $^\circ$.

Two position vectors are given by $\overrightarrow{r_1} = (1, 1, 1)$ and $\overrightarrow{r_2} = (1, -1, 1)$. Find the unit vector in the direction of $\overrightarrow{r_1} \times \overrightarrow{r_2}$.

Two vectors $\vec{A}$ and $\vec{B}$ are at right angles to each other,when

If $\vec A$ and $\vec B$ are perpendicular vectors,where $\vec A = 5\hat i + 7\hat j - 3\hat k$ and $\vec B = 2\hat i + 2\hat j - a\hat k$,then the value of $a$ is: