$\displaystyle b= \frac{1}{4}a$\displaystyle b= \frac{1}{4}a

$\displaystyle b= \frac{1}{3}a$\displaystyle b= \frac{1}{3}a

$\displaystyle b= \frac{1}{2}a$\displaystyle b= \frac{1}{2}a

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