3 replies to this topic

[Soyuz]

Hi there,

 

I am preparing for FE exam and planing to take it this November.  I came across one solved problem (FE Review Manual by Michael Lindeburg, 3^rd edition revised, on page 7-8, problem 2).  It seems to me the solution is not right. Can somebody help me to understand?  Thanks.

 

It is asking what is the first derivative, dy/dx, of the following expression:  (xy)^x = e

 

Solution:

1. ln(xy)^x = lne (I agree)
2. xln xy = 1 (I agree)
3. d/dx (xlnxy) = d/dx (1) (I agree) x (1/xy)(xy'+y)+(1) lnxy = 0 (I don't get this part) Because, it seems "x" front of the "lnxy" is treated like constant.

Edited by Art Montemayor, 11 August 2014 - 01:15 PM.

[MrShorty]

You have a fairly involved combination of product rules and chain rules. Can you walk us through the steps you are taking to get the derivative? I get the same answer as the manual, so I'm guessing that you are missing something in one of the product or chain rules.

 

It might help track the problem down if you make the following substitution: u=xy ---> d/dx (x*ln(u))

[breizh]

Soyuz ,

The calculation is correct and follow the advice given by Mr Shorty .

 

The derivative should give you :

ln(y*x) +x/(y*x) *( y'*x + y) =0

or Ln(y*x) +(y'*x/y+1) =0

 

Breizh

[Ericp]

I want to give FE exam (chemical) in Texas

Can anyone tell me if there are two section ? and how are the two sections separated?

is it 110 questions on all the syllabus mentioned or do they have first general section and then department specific.
And you can call break anytime you want after say 50 questions?