A circular current carrying coil has a radius R. The distance from the centre of the coil on the axis where the magnetic induction will be 18th to its value at the centre of the coil, is
R√3
BcentreBaxis=(1+x2R2)3/2,alsoBaxis=18Bcentre⇒81=(1+x2R2)3/2⇒2=(1+x2R2)1/2⇒4=1+x2R2⇒3=x2R2⇒x2=3R2⇒x=√3R