Hey MathForum,

Im not sure if this is for this section.

The field strength of a magnet(H)at a point on the axis, distance $\displaystyle x$ from its centre, is given by

$\displaystyle H = \frac{M}{2l} \left\{ \frac{1}{(x-l)^2} - \frac{1}{(x+l)^2}\right\}$

where2l= length of magnet andM= moment. Show that, iflis very small compared with $\displaystyle x$, then $\displaystyle H\approx\frac{2M}{x^3}$

How would you solve this question?

Thanks