Consider a solenoid of length and radius R, containing N closely spaced turns and carrying a steady

Consider a solenoid of length ℓ and radius R, containing N closely spaced turns and carrying a steady current I.
(a) In terms of these parameters, find the magnetic field at a point along the axis as a function of distance a from the end of the solenoid.
(b) Show that as ℓ becomes very long, B approaches μ0NI/2! at each end of the solenoid.