איך גוזרים שורש שלישי?

(y = cbrt f(x
(y = f^(1/3) f(x
(y' = 1/3f^(-2/3)(x) * f'(x =

(f^(-2/3)(x) * f'(x
---------------------------- =
3

(f'(x
-----------------
(f^(2/3)(x
-------------------------- =
3
-----------
1

(שיטת האוזן)
(f'(x
-------------------
(3cbrt f^2(x