7. Differentiating Powers of a Function

by M. Bourne

Function of a Function

If y is a function of u, and u is a function of x, then we say

"y is a function of the function u".

Example 1

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In this section:
Chain Rule
Power Rule

Consider the function

y = (5x + 7)12.

If we let u = 5x + 7 (the inner-most expression), then we could write our original function as

y = u12

We have written y as a function of u, and in turn, u is a function of x.

This is a vital concept in differentiation, since many of the functions we meet from now on will be functions of functions, and we need to recognise them in order to differentiate them properly.

Continues below ⇩

Chain Rule

To find the derivative of a function of a function, we need to use the Chain Rule:

`(dy)/(dx) = (dy)/(du) (du)/(dx)`

This means we need to

  1. Recognise `u` (always choose the inner-most expression, usually the part inside brackets, or under the square root sign).
  2. Then we need to re-express `y` in terms of `u`.
  3. Then we differentiate `y` (with respect to `u`), then we re-express everything in terms of `x`.
  4. The next step is to find `(du)/dx`.
  5. Then we multiply `dy/(du)` and `(du)/dx`.

Answer for Example 1

In Example 1,

y = u12, so `(dy)/(du) = 12u^11 = 12(5x+7)^11`

and

u = 5x + 7, so `(du)/(dx) = 5`

So

`dy/dx=(dy)/(du) (du)/(dx) ` `= (12(5x+7)^11)(5) ` `= 60(5x+7)^11`

Example 2

Find `dy/dx` if `y = (x^2+ 3)^5`.

Answer

In this case, we let u = x2 + 3 and then y = u5.

We see that:

  • u is a function of x and
  • y is a function of u.

For the chain rule, we firstly need to find `(dy)/(du)` and `(du)/(dx)`:

`(dy)/(du)=5u^4=5(x^2+3)^4`; and

`(du)/(dx)=2x`

So

`(dy)/(dx)=(dy)/(du)(du)/(dx)`

`=5(x^2+3)^4(2x)`

`=10x(x^2+3)^4`

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Example 3

Find `dy/dx` if `y=sqrt(4x^2-x)`.

Answer

In this case, we let u = 4x2 − x and then `y=sqrtu=u^(1/2)`.

Once again,

  • u is a function of x and
  • y is a function of u.

Using the chain rule, we firstly need to find:

`(dy)/(du)=1/2u^(-1/2)=1/(2sqrtu)=1/(2sqrt(4x^2-x)`

and

`(du)/(dx)=8x-1`

So

`(dy)/(dx)=(dy)/(du)(du)/(dx)`

`=1/(2sqrt(4x^2-x))(8x-1)`

`=(8x-1)/(2sqrt(4x^2-x))`

Note: How does `u^(-1//2)=1/sqrtu`? See:

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The Derivative of a Power of a Function (Power Rule)

An extension of the chain rule is the Power Rule for differentiating. We are finding the derivative of un (a power of a function):

`d/dxu^n=n u^(n-1)(du)/dx`

Example 4

In the case of `y=(2x^3-1)^4` we have a power of a function.

Answer

If we let u = 2x3 - 1 then y = u4.

So now

  • y is written as a power of u; and
  • u is a function of x [ u = f(x) ].

To find the derivative of such an expression, we can use our new rule:

`d/(dx)u^n=n u^(n-1)(du)/(dx`

where u = 2x3 − 1 and n = 4.

So

`(dy)/(dx)=n u^(n-1)(du)/(dx)`

`=[4(2x^3-1)^3][6x^2]`

`=24x^2(2x^3-1)^3`

We could, of course, use the chain rule, as before:

`(dy)/(dx)=(dy)/(du)(du)/(dx`

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Example 5

If `y=1/x`, find `dy/dx`

Answer

We can write `y=1/x` as `y=x^-1`. So:

`dy/dx=(-1)x^-2 = -1/x^2`

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CHALLENGE

Find the derivative of `y=(x^2(3x+1))/(x^4+2)`

Answer

In this example, we have a quotient, where the numerator is a product.

Once again, we let `y=u/v`, where `u=x^2(3x+1)` and `v=x^4+2`.

The quotient formula requires `(du)/(dx)` but u is a product.

Let `u = pq` where p = x2 and q = 3x + 1.

`(du)/(dx)=p(dq)/(dx)+q(dp)/(dx)`

`=(x^2)(3)+(3x+1)(2x)`

`=3x^2+6x^2+2x`

`=9x^2+2x`

We also require

`(dv)/(dx)=4x^3`

So

` (dy)/(dx)=(d(u/v))/(dx)=(v(du)/(dx)-u(dv)/(dx))/(v^2)`

`=((x^4+2)(9x^2+2x)-(x^2)(3x+1)(4x^3))/((x^4+2)^2)`

`=(9x^6+2x^5+18x^2+4x-12x^6-4x^5)/((x^4+2)^2)`

`=(-3x^6-2x^5+18x^2+4x)/((x^4+2)^2)`

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