Minimising potential energy in question 3

[babtridge]

OK so I'm revising for my finals and have spent 14 hrs revising today and cannot for the life of me get the very last part of Qn 3 on the attached link.
It begins "By minimising the potential energy..."

Could someone throw me a lifeline and explain what I need to do because my exam's in less than 12 hours and I'm nearly OD'ing on ProPlus!

Thanks a lot

 

[Rebel]

Ok, if you make a partial derivation respect to z and equal the result to zero, to minimize it; take off with the equality the constant pi and continue having ro, there put the argument that the integrand should be a constant (lambda) -the integration was over rdr - so you have the form requering, there you have actually directly the form of "a" and then go to pick the value required of lambda according to the problem . (put it more formal than me pls, i just taked a look)

 

[babtridge]

Yeah that's the one mate.

 

[Mk]

Sorry, hope you did ok on the test. How'd you do?