Let current in part $A B C$ is $i_1$ and in part $A D C$ is $i_2$

$i=\frac{i l_2}{l_1+l_2}$ (As $A B C$ and $A D C$ part are in parallel connection) and subtended by $A B C$ at centre $O$ will be $=\left(\frac{2 \pi}{l_1+l_2}\right)\left(l_1\right)$

so using $B=\frac{\mu_0 i}{2 a}\left(\frac{\theta}{2 \pi}\right)$

$B=\frac{\mu_0}{2 R}\left(\frac{i l_2}{l_1+l_2}\right) \frac{2 \pi}{\left(l_1+l_2\right)} \frac{\left(l_1\right)}{2 \pi}$