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Derivative Rules

  • Page ID
    219399
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    Derivatives The Easy Way

     Constant Rule and Power Rule

    We have seen the following derivatives:

    1. If f(x) = c, then f '(x) = 0

    2. If f(x) = x, then f '(x) = 1

    3. If f(x) = x2, then f '(x) = 2x

    4. If f(x) = x3, then f '(x) = 3x2

    5. If f(x) = x4, then f '(x) = 4x3

    This leads us the guess the following theorem.

     

    Theorem

          d
                xn  =  nxn-1
         dx 


    Proof:

    We have

              \(\lim\limits_{h \to 0} \frac{(x + h)^n - x^n}{h}   = \lim\limits_{h \to 0} \frac{x^n + nx^{n-1}h + C(n,2)x^{n-2}h^2 + ...  + nxh^{n-1} + h^n - x^n}{h}   \)

            \(= \lim\limits_{h \to 0} \frac{nx^{n-1}h + C(n,2)x^{n-2}h^2 + ...  + nxh^{n-1} + h^n} {h}   \)

            \(= \lim\limits_{h \to 0} \frac{h(nx^{n-1} + C(n,2)x^{n-2}h + ...  + nxh^{n} + h^{n-1}} {h}   \)

            \(= \lim\limits_{h \to 0} \frac{nx^{n-1} + C(n,2)x^{n-2}h + ...  + nxh^{n} + h^{n-1} }{1}  = nx^{n - 1} \)

     


    Applications

    Example

    Find the derivatives of the following functions:

    1. f(x) = 4x3 - 2x100

    2. f(x) = 3x5 + 4x8 - x + 2

    3.  f(x) = (x3 - 2)2

    Solution  

    We use our new derivative rules to find

    1. 12x2 - 200x99

    2. 15x3+32x7-1

    3. First we FOIL to get

              [x6 - 4x3 + 4] ' 

      Now use the derivative rule for powers

              6x5 - 12x2 


    Example:

    Find the equation to the tangent line to 

            y  =  3x3 - x + 4 

    at the point (1,6)

    Solution:

            y'  =  9x2 - 1 

    at x = 1 this is 8. Using the point-slope equation for the line gives

            y - 6  =  8(x - 1) 

    or 

            y  =  8x - 2


    Example:

    Find the points where the tangent line to 

            y  =  x3 - 3x2 - 24x + 3 

    is horizontal.

    Solution:

    We find 

            y'  =  3x2 - 6x - 24

    The tangent line will be horizontal when its slope is zero, that is, the derivative is zero.  Setting the derivative equal to zero gives:

            3x2 - 6x - 24  =  0 

    or

            x2 - 2x - 8  =  0 

    or

            (x - 4)(x + 2)  =  0

    so that 

            x = 4    or    x = -2
     


    Derivative of f(x) = sin(x)
     

     

    Theorem

          d
                sin(x)  =  cos(x)
         dx
              

     


    Proof:  

            \(= \lim\limits_{h \to 0} \frac{sin(x+h) - sin(x)} {h}  = \lim\limits_{h \to 0} \frac{sin(x)cos(h) + sin(h)cos(x) - sin(x)} {h}   \)

            \(= \lim\limits_{h \to 0} [\frac{sin(x)cos(h) - sin(x)} {h} + \frac{sin(h)cos(x)}{h}]   = sin(x)\lim\limits_{h \to 0} [\frac{cos(h) - h} {h}] + cos(x)\lim\limits_{h \to 0} [\frac{sin(h)cos(x)} {h}]   \)

           \( = (sin(x))(0) + (cos(x))(1) = cos(x) \)


     


    d/dx cos(x)

     

    Theorem

           d   
                 cos x  =  -sin x
          dx

     

     


    Back to Derivatives Page

     

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