Infinte number of straight wires each carrying curret I are equally placed as show in the figure. Adjacent wire have currents in opposite direcation. Net magnetic field at point P is :

Explanation

`B_("net")=(mu_(0)I)/(4pi)(sin30^(30)+sin30^(@))hatk[1/d-1/(2d)+1/(3d)-1/(4d)…..+oo]`
(where `d=acos30=(sqrt3a)/2)`
`thereforeB_("net")=(mu_(0)I)/(2sqrt3pia)hatk[1-1/2+1/3-1/4+….oo]`
`=(mu_(0)I)/(2sqrt3pia)"ln"2hatk=(mu_(0)I)/(4pi)("ln"4)/(sqrt3a)hatk`