The magnetic field at the centre of a circular coil of radius r and carrying a current i is B. What is the magnetic field at a distance X=3r r from the centre, on the axis of the coil ?

The magnetic induction at the centre of circular coil is given byB=B0=μ0Ni2r………………….(1)The magnetic' induction at a distance x on the axis of circular coil is given byBX=μ0Nir22r2+x2j3/2=μ0Nir22r2+3r23/2 (∵x=3r)=μ0Nir216r3=μ0Ni16r…………….(2)From eqs. (1) and (2), Bx = B / 8