If f and g are monotonic, is f(g(x))?

  • Thread starter NWeid1
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In summary, if f and g are both increasing functions, then f(g(x)) is also increasing. This can be proven using the definition of an increasing function and the fact that if x<y, then f(x)<f(y) and g(x)<g(y).
  • #1
NWeid1
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Homework Statement


If f and g are both increasing functions, is it true that f(g(x)) is also increasing? Either prove that it is true or five an example that proves it false.


Homework Equations





The Attempt at a Solution


I know that it is indeed also increasing, but I'm unsure how to prove it.
 
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  • #2
It might help to write down the definition of an increasing function
 
  • #3
A function f(x) is increasing at a point x0 if and only if there exists some interval containing x0 such that f(x0) > f(x) for all x in I to the left of x0 and f(x0) < f(x) for all x in the interval to the right of x0.
 
  • #4
NWeid1 said:
A function f(x) is increasing at a point x0 if and only if there exists some interval containing x0 such that f(x0) > f(x) for all x in I to the left of x0 and f(x0) < f(x) for all x in the interval to the right of x0.
How would you express that in terms of calculus?
 
  • #5
Um idk by saying that as x increases, so does the y values.
 
  • #6
Try it without words. Write an equation. Using calculus.
 
  • #7
If x>y then f(x)>f(y)?
 
  • #8
You posted this question in "Calculus and beyond," so use calculus.
 
  • #9
You don't really need calculus here.

Just use that

[tex]x<y~\Rightarrow~f(x)<f(y)[/tex]

and

[tex]x<y~\Rightarrow~g(x)<g(y)[/tex]

Put these two things together.
 

1. What does it mean for a function to be monotonic?

A monotonic function is one that either always increases or always decreases as its input increases. In other words, the function's output either consistently increases or consistently decreases as its input increases.

2. Can f(g(x)) still be monotonic if f and g are both monotonic?

Yes, f(g(x)) can still be monotonic in this case. If both f and g are monotonic, then the composition of the two functions will also be monotonic. This is because the monotonicity of the composition is determined by the monotonicity of the individual functions.

3. What happens if only one of the functions, f or g, is monotonic?

If only one of the functions is monotonic, then the monotonicity of the composition will depend on the specific functions and how they interact with each other. It is possible for f(g(x)) to be monotonic, non-monotonic, or even constant in this case.

4. Are there any specific conditions that must be met for f(g(x)) to be monotonic?

Yes, there are certain conditions that must be met for the composition to be monotonic. One important condition is that the range of g must be contained within the domain of f. Additionally, the monotonicity of f and g must be consistent in the overlapping regions of their domains.

5. Can f(g(x)) be both increasing and decreasing at different intervals?

No, f(g(x)) cannot be both increasing and decreasing at different intervals. If the composition is monotonic, it will either always increase or always decrease. This is because the monotonicity of the composition is determined by the monotonicity of the individual functions, and a function cannot be both increasing and decreasing at the same time.

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