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Derivatives of Composite Functions - Chain Rule

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Estimated time: 1 minutes

Notes

To find the derivative of  f, |
where f(x) = `(2x + 1)^3`
One way is to expand (2x + 1)3 using binomial theorem and find the derivative as a polynomial function as illustrated below.
`d/(dx)`f(x) = `d/(dx) [(2x + 1)^3]`

=`d/(dx) (8x^3 + 12x^2 + 6x + 1)`

= `24x^2 + 24x + 6 `

= `6 (2x + 1)^2` 
Now, observe that  

f(x) = (h o g) (x)

where g(x) = 2x + 1 and h(x) = `x^3`.

Put t = g(x) = 2x + 1. Then f(x) = h(t) = `t^3`. Thus 

`(df)/(dx) = 6(2x + 1)^2 = 3(2x + 1)^2 . 2 = 3t^2 . 2 = (dh)/(dt) . (dt)/(dx)`
The advantage with such observation is that it simplifies the calculation in finding the derivative of, say, `(2x + 1)^100`. We may formalise this observation in the following theorem called the chain rule.

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