very simple questions for experts

How did one arrive at the red-underlined part in the equation (2) from differentiating equation (1)? Please see the picture:

https://imgur.com/a/FZBUaY6

The paper referred here:

Chen, Y., Zhuang, W., Wang, S., Lin, J., Balint, D., & Shan, D. (2012). Investigation of FE model size definition for surface coating application. Chinese Journal of Mechanical Engineering, 25(5), 860-867.

Hello Everybody,

I have created the input file and want to see the table of output , i put the time step, but instead the program gives me only one line,