Hello, my name is Erik and I知 looking for help regarding an explanation (or a proof) of why the following concept is true. (My teacher cannot give me one and neither can the calculus manual, they both simply state that this is just how it is)

problem:

img508.imageshack.us/img508/4498/problemap1.gif

I知 not sure I知 using the correct notation in solving this problem but really what I知 looking for is the proof or explanation why this is possible and why this is how it should be solved.

Thanks

Erik