Derivation of Equation (11) from R Boundary Condition

[Dustinsfl]

http://www.vibsys.put.poznan.pl/journal/2010-24/noga-2.pdf

Equations (9) leads to (11).

How from R boundary condition do we end up with the equation in (11)?

 

[Dustinsfl]

http://www.vibsys.put.poznan.pl/journal/2010-24/noga-2.pdf

Equations (9) leads to (11).

How from R boundary condition do we end up with the equation in (11)?

Ok so I figured out what they have done but what does this equation do for them?