Differentiation & composite functions (2)

In summary: This is nothing special to calculus. It is just using the definition of what a symbol means.In summary, if we are given that dF(y)/dy = f(y), then it is true that dF(h(u))/du = f(h(u)) dh(u)/du. This is due to the application of the chain rule, where y is defined as h(u). Therefore, y and h(u) can be freely replaced by each other in the equation.
  • #1
kingwinner
1,270
0
Suppose we are given that
d
--F(y) = f(y)
dy

Then is it true that
dF(h(u))
-------- = f(h(u)) dh(u)/du ?
du
Why or why not? In particular, I don't understand how to get the red part...

Thanks for explaining!
 
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  • #2
It is basically just application of the chain rule.
Calling y = h(u),

[tex]\frac{dF(y)}{du} = \frac{dF(y)}{dy} \times \frac{dy}{du}[/tex]
so if you call dF(y)/dy = f(y) then you have your identity (just replace y with its definition h(u) again).
 
  • #3
Try this: http://en.wikipedia.org/wiki/Chain_rule

There is a proof about halfway down the page.

If that proof is confusing, then I should think your calculus textbook ought to provide a proof also, which may be easier to follow.

Or you can try to prove it yourself using the limit definition of the derivative.
 
  • #4
CompuChip said:
It is basically just application of the chain rule.
Calling y = h(u),

[tex]\frac{dF(y)}{du} = \frac{dF(y)}{dy} \times \frac{dy}{du}[/tex]
so if you call dF(y)/dy = f(y) then you have your identity (just replace y with its definition h(u) again).

OK, thanks!

But can we replace y by h(u) and vice versa that freely?

Sorry my calculus is a bit rusty now...
 
  • #5
You can replace anything by anything that is equal to it!
 
  • #6
kingwinner said:
OK, thanks!

But can we replace y by h(u) and vice versa that freely?

Sorry my calculus is a bit rusty now...

I defined y to be h(u). So, as HallsOfIvy points out, anywhere you see one you can replace it by the other.
 

Related to Differentiation & composite functions (2)

1. What is differentiation?

Differentiation is a mathematical process that involves finding the rate of change of a function, or its slope, at a specific point. It is a fundamental concept in calculus and is used to solve problems involving rates of change in various fields such as physics, engineering, and economics.

2. What is the difference between differentiation and integration?

While differentiation is the process of finding the slope of a function, integration is the reverse process of finding the function itself from its slope. In other words, differentiation deals with the instantaneous rate of change, while integration deals with the cumulative change over a given interval.

3. What are some common applications of differentiation?

Differentiation has various applications in real-life problems, such as finding the maximum or minimum value of a function, calculating the velocity and acceleration of moving objects, determining the rate of change in financial markets, and optimizing production processes.

4. What is a composite function?

A composite function is a function that is formed by combining two or more functions. It is written as f(g(x)), where the output of one function, g(x), becomes the input of another function, f(x). This allows us to represent complicated relationships between variables in a more simplified form.

5. How do you differentiate a composite function?

To differentiate a composite function, we use the chain rule, which states that the derivative of a composite function is equal to the derivative of the outer function evaluated at the inner function, multiplied by the derivative of the inner function. In other words, we take the derivative of the outer function, leaving the inner function unchanged, and then multiply it by the derivative of the inner function.

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