2.15: (Optional) — Is limx→cf′(x) Equal to f′(c)?
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Consider the function
f(x)={sinx2xif x≠00if x=0
For any x≠0 we can easily use our differentiation rules to find
f′(x)=2x2cosx2−sinx2x2
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 limx→0f′(x)=f′(0). We will see a function for which limx→0f′(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 limx→0f′(x)=f′(0).
Consider the function
f(x)={x2sin1xif x≠00if x=0
For x≠0 we have, by the product and chain rules,
f′(x)=2xsin1x+x2(cos1x)(−1x2)=2xsin1x−cos1x
As |sin1x|≤1, we have
limx→02xsin1x=0
On the other hand, as x tends to zero, 1x goes to ±∞. So
limx→0cos1x=DNE⟹limx→0f′(x)=DNE
We will now see that, despite this, f′(0) is perfectly well defined. By definition
f′(0)=limh→0f(h)−f(0)h=limh→0h2sin1h−0h=limh→0hsin1h=0since |sin1h|≤1
So f′(0) exists, but is not equal to limx→0f′(x), which does not exist.
Now for the good news.
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 limx→cf′(x) exists.
Then f is differentiable at x=c and
f′(c)=limx→cf′(x)
- Proof.
-
By hypothesis, there is a number L such that
limx→cf′(x)=L
By definition
f′(c)=limh→0f(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)=limh→0f′(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)=limh→0f′(xh)=L
In the next example we evaluate f′(0) by applying Theorem 2.15.2.
Let
f(x)={sinx2xif x≠00if x=0
We have already observed above that, for x≠0,
f′(x)=2x2cosx2−sinx2x2=2cosx2−sinx2x2
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 limx→0f′(x). We verify them now.
- We already know, by Lemma 2.8.1, that limh→0sinhh=1. So
limx→0sinx2x2=limh→0+sinhhwith h=x2=1
andlimx→0f(x)=limx→0sinx2x=limx→0xsinx2x2=limx→0x limx→0sinx2x2=0×1=0
and f(x) is continuous at x=0. - The limit of the derivative is
limx→0f′(x)=limx→0[2cosx2−sinx2x2]=2×1−1=1
So, by Theorem 2.15.2, f(x) is differentiable at x=0 and f′(0)=1.