In order to treat with this disgusting problem on the homepage,I have tried some different methods, including a few experiment. Finally I found a really EASY way.
I first expressed the function of the height of the gravity with the parameter 'h'.
Then I expressed the derived function of f(h) (h≥0) and found out the points when the derived function is ZERO, while f(h) is at the smallest value.
GSP is a very interesting and professional softwere which can build geometrical graphs, even the image of a function. So I can find the position of the "SMALLEST-VALUE-POINT" quickly by searching on the visible interface.I got the answer of 2.27.Though it cannot be completely precision, It's enough for this exercise and most other applications.