# Differentiable

Differentiable means that the derivative exists ...

### Example: is x2 + 6x differentiable?

Derivative rules tell us the derivative of x2 is 2x and the derivative of x is 1, so:

Its derivative is 2x + 6

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

### 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.

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

 |x| looks like this:

At x=0 it has a very pointy change!

Does the derivative exist at x=0?

## Testing

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

lim h→0 f(a+h) − f(a) h

### Example (continued)

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

Start with:lim h→0 f(a+h) − f(a) h
f(x) = |x|:lim h→0 |a+h| − |a| h
a=0:lim h→0 |h| − |0| h
Simplify:lim h→0 |h| h

In fact that limit does not exist! To see why, let's compare left and right side limits:

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

The limits are different on either side, so the limit does not exist at x=0

f(x) = |x| is not differentiable at x=0

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 at x=0 even when zoomed in.

## Other Reasons

Here are a few more examples:

 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, unless we exclude x=0.
 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 there.

## 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.

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