Radius of curvature for a magnetic field line

[catheee]

How to prove the radius of curvature at any point on a line?

In a magnetic field, field lines are curves to which the magnetic induction B is everywhere tangetial. By evaluating dB/ds where s is the distance measured along a field line, prove that the radius of curvature at any point on a line is given by

p= B^3 / [ B x( B * del) B]

where do i start with this?? I have no idea what to do

 

[gabbagabbahey]

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