If we take current carrying straight wire of length, say l, and we want to find magnetic field at point p.

Using Ampere circuital law along circle(perpendicular to wire and centre lying on the wire), the circulation of magnetic field is µ_oi

Further, Magnetic field is tangential to this circle every where and also equal.

Hence implication by the law is

B(2πr)= µ_oi

B=  µ_oi\(2πr)

where r à radius of circle

however if use biot savart law here then we find that

B= {µ_oi(cosφ₁-cosφ₂)}\ 4πr

Where φ₁ and φ₂ à angle between line joining point p and ends of wire  

I am not able to understand what mistake I have committed in applying ampere circuital law here 

frnd you hav made no mistake in deriving both the formula   in  biot-savart 's law    put 

as the circle would subtend   angle  from 0  to 180 degrees on the long wire

Simple substitute now values of cos.

After using biot savart law, we will get the mag field as  mui(cos@d@)/4pir  this should be integrated from -@1 to @2

and so the mag field would be mui(sin@1+sin@2)/4pir

and i think ampere circuital law cannot be applied for the finite wire.

Correct me if i m wrong

 But .... It is also the same case with infinite wire!! ??

comparison Ampere circuital law and biot savart law