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Sequence and Derivative

  1. Sep 13, 2015 #1
    I see that derivative of y with respect to x is just like the ratio of y over x.
    But, Why Un (the formula to find nth term) is not the derivative of Sn (the sum of sequence formula) ??

    For example,

    1 2 5 10 -> y = x2+1
    +1 +3 +5 -> y= 2x-1
    +2 +2 -> y=2

    I think that the second level is 2x (since it's the derivative of y = x2+1), but it's not...
    Derivative of y with respect to x is just like how much y will gain with x gaining, right ?

    Is there any way or formula to correlate the derivative of Sn with Un ??
  2. jcsd
  3. Sep 13, 2015 #2


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    Science Advisor
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    Gold Member

    The derivative is a continuous measure of change. You are comparing it with discrete changes between ##x = 0, 1, 2 \ \dots##

    The function ##y = x^2 + 1## is a smooth curve. You are effectively joining up the points ##(0, 1), (1, 2), (2, 5)## with straight lines. That is the difference.

    If, instead, you took ##x = 2, 4, 6 \dots## or ##x = 10, 20, 30 \dots## you would find a bigger discrepancy between the derivative at a point and the slope of the straight line to the next point.

    Or, if you took ##x = 0.1, 0.2, 0.3 \dots## you would find the slope of the straight lines to be closer to the derivative.

    The smaller you take the difference between values of ##x##, the closer you will get to the derivative. The derivative is, in fact, the limit of the slope of these straight lines as the difference between your points get arbitrarily small.
  4. Sep 13, 2015 #3


    Staff: Mentor

    No, it isn't. The derivative of y with respect to x is the ratio of change in y over change in x. IOW, ##\frac{dy}{dx} = \lim_{\Delta x \to 0} \frac {\Delta y}{\Delta x}##, one of several formulations for this derivative.
  5. Sep 13, 2015 #4
    We have U(n) = S(n+1) - S(n)
    which looks similar to the derivative
    dy/dx = lim (h->0) of (y(x+h) - y(x)) /h
    In fact it is the same expression when h=1
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