This question belongs on the Physics Help Forum.
well, using biot sarvart law, we can easily find the expression for magnetic field at the centre of a current carrying coil, we get B=(mu)*i/(2*r)...how to use ampere circuital law to do the same??
It belongs on the physics help forum, because you're likely to get better help there. People on this forum are not necessarily up on the intricacies of E&M, especially in the Pre-University Math Help subforum. You wouldn't publish a physics paper in the TV Guide, would you? I'm saying this for your benefit. If you want good help that's tailored to your situation, then I'm advising you to go to the physics help forum.