Let us consider two angles made by points A and B with point O be θ₁ and θ₂ respectively.

θ₁ = θ₂ = θ

Let distance of the point from the wire

In triangle

Thus induced magnetic field due to AB is given by,

⇒

⇒

⇒

Similarly the induced magnetic field due to current in BC is given as:

Now, magnetic field due to CD = magnetic field due to current in wire AB =B

Also, magnetic field due to current in wire DA = magnetic field due to current in wire BC =B’

∴