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_(n et)=(mu_(0)I)/(4pi)(sin30^(@)+sin30^(@))hat(k)[(1)/(d)(1)/(2d)+(1)/(3d)(1)/(4d).....+oo]`
`(` Where`d=a cos 30=(sqrt(3a))/(2)]`
`:. B_(n et)=(mu_(0)I)/(2sqrt(3)pi a ) hat(k)[1-(1)/(2)+(1)/(3)-(1)/(4)....oo]`
`=(mu_(0)I)/(2sqrt(3)pia)ln 2 hat(k)=(mu_(0)I)/(4pi)(ln4)/(sqrt(3)a)hat(k)`