Hi, thanks for your attention!!!

I have recently read the book, Introduction to Micromechanics and Nanmechanics, written by ShaoFan, Li. This is
really a good book for the freshmen who are interested in
micromechanics area. 

But one problem puzzled me for along time,  the puzzlement i have put it in the attached file.

If you know the answer,please tell me. You kindly reply will be great appreciated! Thank you very much!!!

Email   zengtao1984@gmail.com   zwave1984@hotmail.com