(d) $B = 4\left\lbrack \frac{\mu_{0}}{4\pi}\frac{I}{\left( \frac{1}{2} \right)}\ \left( \sin{45{^\circ}} + \sin{45{^\circ}} \right) \right\rbrack$
$= \frac{\mu_{0}}{4\pi}\frac{2I}{L}\ .\frac{2}{\sqrt{2}}\ $
$B = \frac{\mu_{0}}{\pi}\frac{2\sqrt{2}I}{L}$
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