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:

where u = 2x³ - 1 and n = 4.

So

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