Magnetic fiweld due to current in weire `1` at point `p` distant `r` from the wire is
`B = (mu_(0))/( 4 pi) ( i_(1))/( r )[ cos theta + cos theta]`
`(mu_(0))/( 4 pi) ( i_(1) cos theta)/( r ) (directed perpendicular to the plane of paper , inwards)`
The force exterted due to this magnetic field on current element `i_(2) dl` is
`dF = i_(2) dl B sin 90^(@)`
:. `dF = i_(2)dl [(mu_(0))/( 4 pi) ( i_(1) cos theta)/( r )] = (mu_(0))/( 2 pi r) i_(1)i_(2) dl cos theta`
