$\displaystyle a=- \frac{3}{2}$\displaystyle a=- \frac{3}{2}

$\displaystyle a=- \frac{2}{3}$\displaystyle a=- \frac{2}{3}

$\displaystyle a=- \frac{2}{5}$\displaystyle a=- \frac{2}{5}

$\displaystyle a=- \frac{3}{5}$\displaystyle a=- \frac{3}{5}