(a) $\overrightarrow{B} = {\overrightarrow{B}}_{1} + {\overrightarrow{B}}_{2}$
$\left| {\overrightarrow{B}}_{1} \right| = \left| {\overrightarrow{B}}_{2} \right| = \frac{\mu_{0}i}{4R}$
$\left| \overrightarrow{B} \right| = \sqrt{B_{1}^{2} + B_{2}^{2}}$
$\left| \overrightarrow{B} \right| = \frac{\mu_{0}i}{4R}\sqrt{2} = \frac{\mu_{0}i}{2\sqrt{2}R}$