$\frac{\mu_0 i R^2}{2\left(R^2+x^2\right)^{3 / 2}}=\frac{1}{2 \sqrt{2}} \frac{\mu_0 i}{2 R}$
$2 \sqrt{2} R^3=\left(R^2+x^2\right)^{3 / 2}$
$(2 \sqrt{2})^{2 / 3} R^2=R^2+x^2$
$x^2=2 R^2-R^2=R^2 \Rightarrow x=R$

$(i)\,\,\left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{d\vec l\, \times \,\vec r}}{{{r^3}}}} \right)$
$(ii)\,\, - \left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{d\vec l\, \times \,\vec r}}{{{r^3}}}} \right)$
$(iii)\,\left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{\,\vec r \times d\vec l}}{{{r^3}}}} \right)$
$(iv)\, - \left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{\,\vec r \times d\vec l}}{{{r^3}}}} \right)$