HallsofIvy -I mean finding the length of any part of a rod. It can be L/3 or L/4. Yes, 0 to L/3 is L/3. Sorry for my English...
bigfooted - Thank you. Your way is similar to Pi calculation examples with this method; but is this the only way ? I think, if you draw i can understand better. I...
0_______._________________L
L/3
Suppose that there is a rod has length L. How can you calculate some part of its length from starting point (for example from 0 to L/3)?