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**I had been doing some problems on "Magnetic field due to a current." Now i have one in which one has to find the field at the centre of a regular n-sided polygon. I dont know why i'm not getting it.**

__INTRODUCTION:__**"A regular polygon of n sides is formed by bending a wire of total length 2Πr which carries a current i.(a)Find the magnetic field at the centre of the polygon.(b)By letting n -> infinity, deduce the expression for the magnetic field at the centre of a circular current."**

__THE EXACT PROBLEM:__**The first part is allright.The length of each side is (2Πr)/n. Now using B=(mew)i (cosθ-cosθ)/(4Πd), i got the magnetic field at the centre of the polygon as {(mew)i (n^2) sin(Π/n)tan(Π/n)}/2(Π^2)r. But i have no idea how to get part (b).**

__WHAT MY BRAIN SUGGESTED:__**The magnetic field at the centre of a circular current is (mew)i/2a, where a is the radius of the circle. How do we deduce this from the earlier expression?**

__WHAT I ALSO KNOW:__**I really dont know how to deduce part (b).Please help. Thanks a lot.**

__CONCLUSION:__