Assertion (A): For any non-zero unit vector a, a ·(- a )=(- a ) · a =-1. Reason (R): Angle between a and (- a ) is (π)/2.

Assertion (A) : For any non-zero unit vector $\displaystyle \overrightarrow{\mathrm{a}}, \overrightarrow{\mathrm{a}} \cdot(-\overrightarrow{\mathrm{a}})=(-\overrightarrow{\mathrm{a}}) \cdot \overrightarrow{\mathrm{a}}=-1$. Reason $\displaystyle (R)$ : Angle between $\displaystyle \overrightarrow{\mathrm{a}}$ and $\displaystyle (-\overrightarrow{\mathrm{a}})$ is $\displaystyle \frac{\pi}{2}$.