## Differentiate from first principle(otherwise known as differentialisation)Edit

About supplementary chapters
All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. |

This section and the *differentiation technique* section can be skipped if you are already familiar with calculus/differentiation.

In calculus, differentiation is a very important operation applied to functions of real numbers. To differentiate a function f(x), we simply evaluate the limit

where the means that we let *h* approach 0. However, for now, we can simply think of it as putting *h* to 0, i.e., letting *h* = 0 at an appropriate time. There are various notations for the result of differentiation (called the derivative), for example

and

mean the same thing. We say, f'(x) is the derivative of f(x). Differentiation is useful for many purposes, but we shall not discuss why calculus was invented, but rather how we can apply calculus to the study of generating functions.

It should be clear that if then the above law is important. If g(x) a closed-form of f(x), then it is valid to differentiate both sides to obtain a new generating function.

Also if This can be verified by looking at the properties of limits.

#### Example 1Edit

Differentiate from first principle f(x) where

Firstly, we form the *difference quotient*

We can't set *h* to 0 to evaluate the limit at this point. Can you see why? We need to expand the quadratic first.

We can now factor out the *h* to obtain now

from where we can let *h* go to zero safely to obtain the derivative, 2*x*. So

or equivalently:

#### Example 2Edit

Differentiate from first principles, p(x) = x^{n}.

We start from the difference quotient:

By the binomial theorem, we have:

The first *x*^{n} cancels with the last, to get

Now, we bring the constant 1/*h* inside the brackets

and the result falls out:

**Important Result**

If

then

As you can see, differentiate from first principle involves working out the derivative of a function through algebraic manipulation, and for that reason this section is algebraically very difficult.

#### Example 3Edit

Assume that if

then

Differentiate

**Solution** Let

#### Example 4Edit

Show that if

**Solution**

#### Example 5Edit

Differentiate from first principle

**Solution**

#### ExercisesEdit

1. Differentiate

2. Differentiate

3. Differentiate from first principle

4. Differentiate

5. Prove the result assumed in example 3 above, i.e. if

*f*(*x*)=*g*(*x*)+*h*(*x*)

then

*f*′(*x*)=*g*′(*x*)+*h*′(*x*).

*Hint: use limits*.

### Differentiating f(z) = (1 - z)^nEdit

We aim to derive a vital result in this section, namely, to derive the derivative of

where *n* ≥ 1 and *n* an integer. We will show a number of ways to arrive at the result.

#### Derivation 1Edit

Let's proceed:

expand the right hand side using binomial expansion

differentiate both sides

now we use

and there are some cancelling

take out a common factor of -n, and recall that 1! = 0! = 1 we get

let j = i - 1, we get

but this is just the expansion of (1 - z)^{n-1}

#### Derivation 2Edit

Similar to Derivation 1, we use instead the definition of a derivative:

expand using the binomial theorem

factorise

take the limit inside (recall that [Af(x)]' = Af'(x) )

the inside is just the derivative of z^{i}

exactly as derivation 1, we get

**Example** Differentiate (1 - z)^{2}

**Solution 1**

- f(z) = (1 - z)
^{2}= 1 - 2z + z^{2} - f'(z) = - 2 + 2z
- f'(z) = - 2(1 - z)

**Solution 2** By the result derived above we have

- f'(z) = -2(1 - z)
^{2 - 1}= -2(1 - z)

#### ExercisesEdit

Imitate the method used above or otherwise, differentiate:

1. (1 - z)^{3}

2. (1 + z)^{2}

3. (1 + z)^{3}

4. (Harder) 1/(1 - z)^{3} (Hint: Use definition of derivative)

### Differentiation techniqueEdit

We will teach how to differentiate functions of this form:

i.e. functions whose reciprocals are also functions. We proceed, by the definition of differentiation:

#### Example 1Edit

by

where *g* is a function of *z*, we get

which confirmed the result derived using a counting argument.

#### ExercisesEdit

Differentiate

1. 1/(1-z)^{2}

2. 1/(1-z)^{3}

3. 1/(1+z)^{3}

4. Show that (1/(1 - z)^{n})' = n/(1-z)^{n+1}

### Differentiation applied to generating functionsEdit

Now that we are familiar with differentiation from first principle, we should consider:

we know

differentiate both sides

therefore we can conclude that

Note that we can obtain the above result by the substituion method as well,

letting z = x^{2} gives you the require result.

The above example demonstrated that we need not concern ourselves with difficult differentiations. Rather, to get the results the easy way, we need only to differentiate the basic forms and apply the substitution method. By basic forms we mean generating functions of the form:

for n ≥ 1.

Let's consider the number of solutions to

for a_{i} ≥ 0 for i = 1, 2, ... n.

We know that for any *m*, the number of solutions is the coefficient to:

as discussed before.

We start from:

differentiate both sides (note that 1 = 1!)

differentiate again

and so on for (n-1) times

divide both sides by (n-1)!

the above confirms the result derived using a counting argument.