Infinite number of straight wires each carrying current I are equally placed as shown in the figure Adjacent wires have current in opposite direction Net magnetic field at point `P` is
.

Explanation

`B_("net") = (mu_0 I)/(4pi) (sin 30^@ + sin 30^@) hatk [ 1/d (1)/(2d) + (1)/(3d) (1)/(4d) ….. + oo ]`
( where `d = a cos 30 = (sqrt3a)/(2) ` )
` = (mu_0 I)/(2 sqrt3 pi a) ln 2 hatk = (mu_0 I)/(4pi) (ln 4)/(sqrt3 a) hatk `