Differentiable

Differentiable means that the derivative exists ...

Example: is x2 + 6x differentiable?

Its derivative is 2x + 6

(Because Derivative rules tell us the derivative of x2 is 2x and the derivative of x is 1.)

So yes! x2 + 6x is differentiable.

... and it must exist for every value in the function's domain.

Domain

In its simplest form the domain is
all the values that go into a function

doman and range

Example (continued)

When not stated we assume that the domain is the Real Numbers.

For x2 + 6x, its derivative of 2x + 6 exists for all Real Numbers.

So we are still safe ... x2 + 6x is differentiable.

But what about this:

Example: The function f(x) = |x| (absolute value):

|x| looks like this:   Absolute Value function

At x=0 it has a very pointy change!

Does the derivative exist at x=0?

Testing

We can test any value "c" by finding if the limit exists:

lim
h→0
f(c+h) − f(c)
h

 

Example (continued)

Let's calculate the limit for |x| at the value 0:

lim
h→0
|0+h| − |0|   =   lim
h→0
|h| − 0   =   lim
h→0
|h|
h h h

 

The limit does not exist

 

To see why, let's compare left and right side limits:

From Left Side:   From Right Side:
lim
h→0
|h|   =  −1
h
 
lim
h→0+
|h|   =  +1
h

The limits are different on either side, so the limit does not exist.

 

So the function f(x) = |x| is not differentiable

A good way to picture this in your mind is to think:

As I zoom in, does the function tend to become a straight line?

The absolute value function stays pointy even when zoomed in.

Other Reasons

Here are a few more examples:

Floor function  

The Floor and Ceiling Functions are not differentiable at integer values, as there is a discontinuity at each jump. But they are differentiable elsewhere.

 
 

The Cube root function x(1/3)

Its derivative is (1/3)x-(2/3) (by the Power Rule)

At x=0 the derivative is undefined, so x(1/3) is not differentiable.

 

At x=0 the function is not defined so it makes no sense to ask if they are differentiable there.

To be differentiable at a certain point, the function must first of all be defined there!

 

As we head towards x = 0 the function moves up and down faster and faster, so we cannot find a value it is "heading towards".

So it is not differentiable.

 

Different Domain

But we can change the domain!

Example: The function g(x) = |x| with Domain (0,+∞)

The domain is from but not including 0 onwards (all positive values).

Which IS differentiable.

(And I am "absolutely positive" about that.)

So the function g(x) = |x| with Domain (0,+∞) is differentiable.

We could also restrict the domain in other ways to avoid x=0 (such as all negative Real Numbers, all non-zero Real Numbers, etc).

 

Why Bother?

Because when a function is differentiable we can use all the power of calculus when working with it.

Continuous

When a function is differentiable it is also continuous.

Differentiable Continuous

But a function can be continuous but not differentiable. For example the absolute value function is actually continuous (though not differentiable) at x=0.