= lim h → 0 3 x 2 h + 3 x h 2 + h 3 h In this step the x 3 terms have been cancelled, leaving only terms containing h. All other terms contain powers of h that are two or greater. d d x ( x 3 ) = lim h → 0 ( x + h ) 3 − x 3 h = lim h → 0 x 3 + 3 x 2 h + 3 x h 2 + h 3 − x 3 h Notice that the first term in the expansion of ( x + h ) 3 is x 3 and the second term is 3 x 2 h. = lim h → 0 ( 3 x 2 + 3 x h + h 2 ) After cancelling the common factor of h, the only term not containing h is 3 x 2. = lim h → 0 h ( 3 x 2 + 3 x h + h 2 ) h Factor out the common factor of h. For this function, both f ( x ) = c f ( x ) = c and f ( x + h ) = c, f ( x + h ) = c, so we obtain the following result:ĭ d x ( x 3 ) = lim h → 0 ( x + h ) 3 − x 3 h = lim h → 0 x 3 + 3 x 2 h + 3 x h 2 + h 3 − x 3 h Notice that the first term in the expansion of ( x + h ) 3 is x 3 and the second term is 3 x 2 h. We first apply the limit definition of the derivative to find the derivative of the constant function, f ( x ) = c. To find derivatives of polynomials and rational functions efficiently without resorting to the limit definition of the derivative, we must first develop formulas for differentiating these basic functions. The functions f ( x ) = c f ( x ) = c and g ( x ) = x n g ( x ) = x n where n n is a positive integer are the building blocks from which all polynomials and rational functions are constructed. In this section, we develop rules for finding derivatives that allow us to bypass this process. The process that we could use to evaluate d d x ( x 3 ) d d x ( x 3 ) using the definition, while similar, is more complicated. For example, previously we found that d d x ( x ) = 1 2 x d d x ( x ) = 1 2 x by using a process that involved multiplying an expression by a conjugate prior to evaluating a limit. 3.3.6 Combine the differentiation rules to find the derivative of a polynomial or rational function.įinding derivatives of functions by using the definition of the derivative can be a lengthy and, for certain functions, a rather challenging process.
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