Find the derivative of the function f(x)=-3(2-9x)^(1/4)

[Cafe au lait]

I have:

f'(x)=-3[1/4(2-9x)(-9)]+0(2-9x)^(1/4)
=-3[-9/4(2-9x)]
=-3(81/4x-9/2)
=-243/4x+27/2

Did I do that right?

 

[tkhunny]

No good. Two things wrong with this one.

1) One is just notation. That "1/4" is very badly noted. Put it in parentheses. Also the "9/4" right after.
2) Why on Earth did you use a Product Rule? Rule out constants before you start. There is no product rule necessary on this one. You can do it if you want, I suppose, but I absolutely would NOT suggest it.

 

[Cafe au lait]

I got it!
f'(x)=(-3/4)(2-9x)^(-3/4)(-9)
=27/[4(2-9x)^(3/4)]

Yeah, product rule complicated things.

 

[tkhunny]

Excellent.