Mathematics

Problem

Let non-zero vectors

\mathbf{r}

and

\mathbf{s}

satisfy

|\mathbf{r}|=|\mathbf{s}|

and

|\mathbf{r}+\mathbf{s}|=\sqrt{3}\,|\mathbf{r}-\mathbf{s}|

. The angle between

\mathbf{r}

and

\mathbf{s}

is a)

120^{\circ}

30^{\circ}

60^{\circ}

90^{\circ}