Homework Help: Differentiation of exponential

1. Oct 22, 2009

chaslltt

1. The problem statement, all variables and given/known data

y = x^(18x^-4)

2. Relevant equations

chain rule
dy/dx a^x = a^x ln a

3. The attempt at a solution
first i used the second equation from above to get
x^(18/x^4)*ln(x)
then i use the chain rule to get
x^(18/x^4)*ln(x)*(-72x^-5)
the computer program i am using is saying this is wrong and the only thing i can think of that is wrong is the fact I may not be able to use the one formula since x is not a constant but if this is true then i have no idea what to do

2. Oct 22, 2009

foxjwill

Try taking the log of both sides and using implicit differentiation.

3. Oct 22, 2009

chaslltt

alright so then i should get
ln y = ln (x^(18/x^4))
1/y = 18/x^4 ln x
= y 18/x^4*1/x
= y 18/x^5
= x^(18/x^4)*18/x^5
?

4. Oct 22, 2009

chaslltt

well the computer told me that solution is wrong also so any other advice would be helpful

5. Oct 22, 2009

Office_Shredder

Staff Emeritus
This is only true if a is a constant! Do you know how to differentiate xx? Hint: Try writing it as ef(x) for some function f(x). The same technique can be applied here.

It doesn't look like you took the derivative of the right hand side in your other post correctly

6. Oct 22, 2009

chaslltt

does this look right
ln y= 18/x^4 ln x
1/y= 18/x^5 + -72x^-5 ln x
= y 18/x^5 + -72x^-5 ln x
then just plug in y ?

7. Oct 23, 2009

foxjwill

yep!

8. Oct 23, 2009

HallsofIvy

You've forgotten the "dy/dx"!

and what is this equal to?

If $ln y= (18/x^4)ln(x)$
then [iitex](1/y)(dy/dx)= (18/x^5) -72x^{-5}ln x[/itex]
Now solve for dy/dx by multiplying both sides by y.
You might not be required to substitute for y. It depends on exactly what is wanted.