Chain rule for product of functions

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Discussion Overview

The discussion revolves around the differentiation of a composite function defined as f(g(x)) = h(x) * g(x). Participants explore the application of the product rule and the chain rule to find df/dx, leading to confusion about the definitions and roles of the functions involved.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates df/dx using the product rule and arrives at g(x)h'(x) + h(x)g'(x), while another uses the chain rule and finds df/dx = h(x) * g'(x), leading to confusion about the correct approach.
  • Participants discuss the definition of f(y) and its derivative, with some suggesting that f(y) should equal h(y)g(y), which is later challenged.
  • There is uncertainty about the existence of two different functions, one being f(x) = h(x) * g(x) and the other being f(g(x)).
  • One participant proposes defining a new function k(x) = f(g(x)) = h(x)g(x) to clarify the relationship, but confusion persists regarding the composition of functions.
  • Another participant suggests setting y = g(x) to simplify the expression, leading to a new formulation for f(y) that incorporates the inverse function g^{-1}(y).
  • Participants express difficulty in understanding the implications of differentiating both sides of the equation and how to derive the relationship between the derivatives.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct interpretation of the functions involved or the proper application of differentiation rules. Confusion and differing viewpoints persist throughout the discussion.

Contextual Notes

There are limitations in the clarity of function definitions and the assumptions made about the relationships between f, g, and h. The discussion also reflects unresolved mathematical steps and varying interpretations of the composition of functions.

smath42
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Here is a simple question :

let f(g(x)) = h(x)*g(x).

I want to calculate df/dx.

If I use the product rule, I get g(x)h'(x) + h(x)g'x).

Now if I use the composition/chain rule, I get

df/dx = df/dg * dg/dx = h(x) * g'(x) which is different.

I guess my df/dg = h is wrong, but I can't see what it should be.

Any help is welcome:-)
 
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I don't want to double up on ##x##, so let's use ##y##.
What is ##f(y)## in the question? What is it's derivative?
 
that's where I'm confused...
my first thought that in f(g(x)) f(y) is the function that multiplies y by h(x). therefore if I derivate with respect to y, I get h(x). Which apparently is wrong...

no your question replaces x by y, which in fact doesn't really change anything so I'd say
f(y) obviously equals to h(y)g(y) so its derivative is h'(y)g(y)+g'(y)h(y)...

now I'm again confused
 
smath42 said:
f(y) obviously equals to h(y)g(y)

This is wrong.
 
humm... so you've seen where my problem is... what's f ?
 
are there 2 f ? the one if f(x) = h(x)*g(x) and the one if f(g(x)) ?
 
smath42 said:
that's where I'm confused...
my first thought that in f(g(x)) f(y) is the function that multiplies y by h(x). therefore if I derivate with respect to y, I get h(x). Which apparently is wrong...

no your question replaces x by y, which in fact doesn't really change anything so I'd say
f(y) obviously equals to h(y)g(y) so its derivative is h'(y)g(y)+g'(y)h(y)...

now I'm again confused

Yes, you've got yourself into a terrible tangle here! If you define another function:

##k(x) = f(g(x)) = h(x)g(x)##

You may be able to see the problem. ##f(x) \ne k(x)##
 
OK I see that now, but I'm still in trouble to calculate df/dg. I'd like to say f(u) = h(x)*u but then again I end up with h(x) for df/dg
 
Instead of trying ##k(x) = (f \circ g)(x) = h(x) \cdot g(x)## start with something simpler ##k(x) = (f \circ g)(x) = (l \circ g) (x) \cdot g(x)##
 
  • #10
hum...
then here too, I can't find what l is although I see l(g(x)) should be h(x)
 
  • #11
Could you guys explain me what is f here please.
I don't know why but I really have trouble seing where is the composition here
 
  • #12
smath42 said:
Could you guys explain me what is f here please.
I don't know why but I really have trouble seing where is the composition here

It's easier if you set ##y = g(x)##, so that ##x = g^{-1}(y)##:

##f(y) = f(g(x)) = h(x)g(x) = h(g^{-1}(y))y##
 
  • #13
hum ok I can now do

f'(y) = h(g^{-1}(y)) + yg^{-1}'(y)h'(g^{-1}(y)) = h(x) + yh'(g^{-1}(y))/g'(g^{-1}(y))
then g'df/dy = g'h+hg'

ok... thanks !
 
  • #14
smath42 said:
hum ok I can now do

f'(y) = h(g^{-1}(y)) + yg^{-1}'(y)h'(g^{-1}(y)) = h(x) + yh'(g^{-1}(y))/g'(g^{-1}(y))
then g'df/dy = g'h+hg'

ok... thanks !

You could have got that directly from the initial equation:

##f(g(x)) = h(x)g(x) \ \Rightarrow \ f'(g(x))g'(x) = h'(x)g(x) + h(x)g'(x)##
 
  • #15
I'm sorry I must have a very slow mind today but I don't see exactly why the implication and the second equality.
 
  • #16
smath42 said:
I'm sorry I must have a very slow mind today but I don't see exactly why the implication and the second equality.

Simply differentiate both sides wrt ##x##.
 
  • #17
ah ok :-D
I was trying to see how one could find h'g+hg' from f'(g)*g' "only"
 

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