Concept of the Derivative - AP Calculus AB

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Question

Evaluate:

The notation is alluding to the fact that the limit is a function of , not necessarily a "number."

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Answer

We are not going to evaluate this directly. Looking at the function, it seems very similar to the definition of a derivative of some kind of trigonometric function. We will look at the definition of a derivative to find a function that would have a derivative defined by this limit.

In otherwords, we wish to identify a function such that its' derivative is the function .

Let's find such that:

Compare corresponding terms in the numerators in the above expressions.

By inspection, these terms clearly indicate that our function must be of the form:

                                                                                                                                 Side note

If your confused by the inclusion of the arbitrary constant , note that when we differentiate this function the constant term will vanish since the derivative of a constant is zero. You can also see that in applying the definition of a derivative, if I included the constant in the two terms in the numerator,

the constant "C" would vanish when we subtract the latter from the former, .

Therefore, even if you didn't consider the constant when working out the function, it would not have changed the result

                                                                                                                                 Because we know that

,

Simply differentiate to find ,

Therefore,

Or to put it another way,

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