With some functions and expressions, differentiation is a long process. e.g. Differentiating the function h(x) = (2x + 3)5 first needs a long expansion of the brackets.
If we think of h(x) = (2x + 3)5 as a combination of two functions with g(x) = 2x + 3 being inside the function f(x) so that
f(x) = (g(x))5. This is known as a function of a function or a composite function f o g.
To differentiate a function of a function we use the Chain Rule.
Chain Rule Formula
This states:
|
(f o g )'(x) = f '(g(x)) . g'(x)
|
The Chain Rule looks complex, but with practice, which is essential, it should become more straightforward.
Example For our example, to differentiate h(x) = (2x + 3)5:
Let f(x) = x5 and g(x) = 2x + 3
Therefore, f '(x) = 5x4 and g '(x) = 2
h '(x) = (f o g )'(x) = f '(g(x)) . g'(x)
= 5(2x + 3)4 . 2
= 10(2x + 3)4
Alternative Chain Rule formula
Another way to differentiate a composite function is to use 
notation, which behaves like fractions.
The Chain Rule states:
 |
Example For our example, to differentiate y = (2x + 3)5
= 2 and
= 5u4
= 5u4 . 2
Roots and Negative Indices using the Chain Rule
As with most differentiation, rational functions and expressions involving roots must be written in index form before differentiation.
Example 1
Differentiate 
Example 2
Differentiate f(x) = √(3x2 + 2)
