Question : Buckling calculation using Tetmajers formula

I'm trying to calculate required diameter, buckling length or buckling force when
just one of them are unknown.

I have done this with the Euler approach sucessfully but run into problems
trying the same with Tetmajers approach.
Please see the attachment for more information.
Attachments:
 
Trying to solve for lk (buckling length)
 

Answer : Buckling calculation using Tetmajers formula

My bad, yes.. It is -ve :-)
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