What is the magnetic field induction at the centre of line joining the two poles of a horse shoe magnet of poles strength ‘m’ and separated by a distance ‘a’ is

The resultant magnetic field at the center of the magnet is                                BR=BN+BS But , BN=BS=μ04πmr2 = μ04πm(a2)2 = 4μ04πma2 ∴BR= 4μ04πma2+ 4μ04πma2 ⇒BR=8μ04πma2