$\displaystyle a_3= \frac{1}{2}$\displaystyle a_3= \frac{1}{2}

$\displaystyle a_3= -\frac{1}{2}$\displaystyle a_3= -\frac{1}{2}

$\displaystyle a_3= \frac{3}{8}$\displaystyle a_3= \frac{3}{8}

$\displaystyle a_3= -\frac{3}{8}$\displaystyle a_3= -\frac{3}{8}