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**scorpio1** Thanks for that, i managed to put in other steps to what you said and it seems right...I just need a little explanation about something, because although it is right (i had the math lecturer look at it) I don't actually understand why it is right, or where certain things come from (lecturer wasn't too clear on that...)

For example you can take:

lim h->0 (f(a+h)-f(a) / h),and you also take:

lim h->0 (f(a)-f(a-h) / h)

Where do these two come from? I mean the first one is just the quotient derivative as we're taught it, simply replacing x with a here though...but the second one?

How do you get the second one? Is it the limit as you approach from the left or something? (This is what i thought, but apprently that's wrong?...)

If someone can please explain what that second equation is and why it's equal to the first, and hence equal to f ' (a)?...

Thanks in advance once again!