Please help in solving this problem. I am unable to find a correct approach.
Answer given in textbook:
You may derive that for a solenoid with a finite length, the magnetic field along its axis equals to
where \theta_1 and \theta_2 are as denoted in the pic below.
In other words,
Also, using \displaystyle\oint\vec B\cdot d\vec S=0 for a small cylinder of thickness dz and radius r, you may find that
I think this info would be helpful for you to tackle this problem.
Thank you, I’ll try again.