Solution 11.

Answer.      is valid for    .   

Solution.   Use the known series      which is valid for  .   

Make the substitutions    and get:  

                       which is valid for  .   

                       which is valid for  .   

Then

                       
                    

Therefore,      is valid for    .   

We are done.   

Aside.  We can let Mathematica double check our work.

This solution is complements of the authors.

(c) 2008 John H. Mathews, Russell W. Howell