Magnetic fields at two points on the axis of a circular coil at a distance of 0.05 m and 0.2 m from the centre are in  the ratio 8:1. The radius of coil is________

Bp=μ0i2r2r2+x232B0.05=μ0i2×r2r2+(0.05)232……(1)B0.2=μ0i2×r2r2+(0.2)232…..(2)(1)(2)B0.05B0.2=r2+(0.2)2r2+(0.05)232⇒8123=r2+(0.2)2r2+(0.05)2⇒4r2+(0.05)2=r2+(0.2)2⇒3r2=(0.2)2−4×(0.05)2=(0.2)2−(2×0.05)2⇒3r2=(0.2)2−(0.1)2=0.04−0.01⇒r2=0.033=0.01⇒r=0.1m