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2.15: (Optional) — Is limxcf(x) Equal to f(c)?

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Consider the function

f(x)={sinx2xif x00if x=0

For any x0 we can easily use our differentiation rules to find

f(x)=2x2cosx2sinx2x2

But for x=0 none of our usual differentation rules apply. So how do we find f(0)? One obviously legitimate strategy is to directly apply the Definition 2.2.1 of the derivative. As an alternative, we will now consider the question “Can one find f(0) by taking the limit of f(x) as x tends to zero?”. There is bad news and there is good news.

  • The bad news is that, even for functions f(x) that are differentiable for all x, f(x) need not be continuous. That is, it is not always true that limx0f(x)=f(0). We will see a function for which limx0f(x)f(0) in Example 2.15.1, below.
  • The good news is that Theorem 2.15.2, below provides conditions which are sufficient to guarantee that f(x) is differentiable at x=0 and that limx0f(x)=f(0).
Example 2.15.1

Consider the function

f(x)={x2sin1xif x00if x=0

For x0 we have, by the product and chain rules,

f(x)=2xsin1x+x2(cos1x)(1x2)=2xsin1xcos1x

As |sin1x|1, we have

limx02xsin1x=0

On the other hand, as x tends to zero, 1x goes to ±. So

limx0cos1x=DNElimx0f(x)=DNE

We will now see that, despite this, f(0) is perfectly well defined. By definition

f(0)=limh0f(h)f(0)h=limh0h2sin1h0h=limh0hsin1h=0since |sin1h|1

So f(0) exists, but is not equal to limx0f(x), which does not exist.

Now for the good news.

Theorem 2.15.2

Let a<c<b. Assume that

  • the function f(x) is continous on the interval a<x<b and
  • is differentiable at every x in the intervals a<x<c and c<x<b and
  • the limit limxcf(x) exists.

Then f is differentiable at x=c and

f(c)=limxcf(x)

Proof.

By hypothesis, there is a number L such that

limxcf(x)=L

By definition

f(c)=limh0f(c+h)f(c)h

By the Mean Value Theorem (Theorem 2.13.5 ) there is, for each h, an (unknown) number xh between c and c+h such that f(xh)=f(c+h)f(c)h. So

f(c)=limh0f(xh)

As h tends to zero, c+h tends to c, and so xh is forced to tend to c, and f(xh) is forced to tend to L so that

f(c)=limh0f(xh)=L

In the next example we evaluate f(0) by applying Theorem 2.15.2.

Example 2.15.3

Let

f(x)={sinx2xif x00if x=0

We have already observed above that, for x0,

f(x)=2x2cosx2sinx2x2=2cosx2sinx2x2

We use Theorem 2.15.2 with c=0 to show that f(x) is differentiable at x=0 and to evaluate f(0). That theorem has two hypotheses that we have not yet verified, namely the continuity of f(x) at x=0, and the existence of the limit limx0f(x). We verify them now.

  • We already know, by Lemma 2.8.1, that limh0sinhh=1. So

    limx0sinx2x2=limh0+sinhhwith h=x2=1

    and

    limx0f(x)=limx0sinx2x=limx0xsinx2x2=limx0x limx0sinx2x2=0×1=0

    and f(x) is continuous at x=0.
  • The limit of the derivative is

    limx0f(x)=limx0[2cosx2sinx2x2]=2×11=1

So, by Theorem 2.15.2, f(x) is differentiable at x=0 and f(0)=1.


This page titled 2.15: (Optional) — Is limxcf(x) Equal to f(c)? is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Joel Feldman, Andrew Rechnitzer and Elyse Yeager via source content that was edited to the style and standards of the LibreTexts platform.

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