Question

For the following function y = f(x) = 4x^3 - 5


I worked out this Calculus problem, but I keep getting 1/-432 for part c. Can anyone work out the problem step-by-step so I can understand what I’m doing wrong? It would be greatly appreciated.

Answer 1

The answer for each of the differentiation are; f'(-6) = 432;  f⁻¹(y) = ∛((y + 5)/4); df⁻¹/dy = 1/432

How to differentiate functions?

1) f(x) = 4x³ - 5

We will differentiate it to get;

f'(x) = df/dx = 12x²

f'(-6) = 12(-6)²

f'(-6) = 432

2) We are given the formula;

x = f⁻¹(y)

Thus;

y = 4x³ - 5

4x³ = y + 5

x³ = (y + 5)/4

x = ∛((y + 5)/4)

f⁻¹(y) = ∛((y + 5)/4)

c)  f⁻¹(y) = ∛((y + 5)/4)

df⁻¹/dy = -1/12y²

At f(-6), we have;

df⁻¹/dy = 1/432

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