Basic question: First derivative test to detect whether a function is decreasing

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Discussion Overview

The discussion revolves around the first derivative test for determining whether a function is decreasing on a specified interval [a,b]. Participants explore definitions, alternative methods, and examples related to this concept.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that if the first derivative of a function f is negative on [a,b], then the function is indeed decreasing on that interval.
  • Another participant agrees with the assertion but emphasizes that negative derivative at a point indicates decreasing behavior for differentiable functions.
  • A different participant points out that the original question seeks alternative arguments for demonstrating that a function is decreasing over an interval.
  • One participant suggests using the definition of increasing functions to argue about decreasing functions, providing an example with f(x) = x^2 to illustrate the concept of increasing behavior.
  • Another participant proposes a formal definition stating that a function f is decreasing on [a,b] if for all x1, x2 in [a,b], x1 < x2 implies f(x1) ≥ f(x2).

Areas of Agreement / Disagreement

Participants generally agree on the definition of decreasing functions in relation to the first derivative. However, there is no consensus on whether additional methods exist to demonstrate decreasing behavior, as some participants suggest alternative approaches while others focus on the derivative test.

Contextual Notes

The discussion does not resolve the question of whether there are universally accepted alternative methods to show that a function is decreasing, leaving some assumptions and definitions potentially unaddressed.

seeker101
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If the first derivative of a function f from R to R is negative on [a,b], it IS right to say that the function is decreasing on [a,b] right?

Are there any other ways of showing that the function is decreasing on [a,b]?
 
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Hi seeker101! :smile:
seeker101 said:
If the first derivative of a function f from R to R is negative on [a,b], it IS right to say that the function is decreasing on [a,b] right?

Yes.
Are there any other ways of showing that the function is decreasing on [a,b]?

No … for a differentiable R->R function, negative derivative at a point is the same as decreasing. :wink:
 
That's correct as a definition. But, what the OP is asking may be whether there are other ways of arguing that a function is decreasing over an interval.
 
Of course there are other ways to show a function is increasing (the same argument can be applied to decreasing) on an interval. For example, using the definition of increasing: if x,y are in [a,b] then show y > x implies f(y) >= f(x).

Example: Show that f(x) = x^2 is increasing on [0,2].
Let x and y belong to [0,2] with y > x. Then we can write y = x + e, for some e>0. Then f(y) = (x+e)^2 = x^2 + 2xe + e^2 > f(x) = x^2, since e^2 is >0 and 2xe is >0.
 
how about a function f is decreasing on [a,b] if [tex](\forall x_1,x_2\in [a,b])[/tex] [tex]x_1<x_2 \implies f(x_1) \ge f(x_2)[/tex]
 

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