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## Calculus, all content (2017 edition)

### Course: Calculus, all content (2017 edition)>Unit 2

Lesson 15: Radical functions differentiation (intro)

Sal differentiates f(x)=-4∛x, and evaluates the derivative at x=8. This can actually be done quite easily using the Power rule!

## Want to join the conversation?

• Why could we have not just cubed -4 and cube root of X? I thought it would work, but I got -64.
• I guess I got your mistake. I guess you did this:

y = (-4) * (cuberoot of x) || raise every term to the power of 3

=> y^3 = (-4^3) * (x)
= y^3 = -64 * (x)

And here is your problem: you finally erased the cube root, but now you have y raised to the third power on the left hand side. It would be incorrect to simply work with y^3 here. I guess you forgot that you raised both sides of the equation.

But it's fine to make mistakes like these, because it shapes your skills.