Vector Algebra

If \(\overline{\mathrm{e}}_1, \overline{\mathrm{e}}_2\) and \(\overline{\mathrm{e}}_1+\overline{\mathrm{e}}_2\) are unit vectors, then the angle between \(\overline{\mathrm{e}}_1\) and \(\overline{\mathrm{e}}_2\) is