How Derivatives Affect the Graph (Proving Question)

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Homework Help Overview

The discussion revolves around the properties of concavity in the context of product functions, specifically examining the conditions under which the product of two functions, f and g, remains concave upward. The problem includes two parts: one for positive, increasing functions and another for decreasing functions.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the conditions under which the product function fg is concave upward, discussing the implications of the derivatives of f and g. There is an attempt to analyze the second derivative of the product function and its positivity under different conditions. Questions arise about the interpretation of increasing and decreasing functions and their derivatives.

Discussion Status

The discussion is ongoing, with participants questioning the assumptions about the behavior of the derivatives of f and g. Some guidance has been offered regarding the interpretation of increasing and decreasing functions, but there is no explicit consensus on the implications for the concavity of the product function.

Contextual Notes

Participants note the definitions of increasing and decreasing functions based on their first derivatives, and there is a mention of a specific example (f = 1/x) to challenge assumptions about concavity. The original poster expresses uncertainty regarding the conditions for concavity in the second part of the problem.

cjaylee
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Homework Statement



(a) If f and g are positive, increasing, concave upward functions on I, show that the product function fg is concave upward on I.

(b)Show that part (a) remains true if f and g are both decreasing.

Homework Equations



-

The Attempt at a Solution



(a)
f>0, f'>0, f''>0
g>0, g'>0, g''>0

(fg)'=f'g+fg'>0
(fg)''=f''g+2f'g'+fg''>0 and so fg is concave upward on I.

(b)
f'<0, f''<0
g'<0, g''<0

(fg)''=f''g+2f'g'+fg''

I have 2f'g'>0 but f''g <0 and fg''<0. Then I'm stuck here. The answer mentioned that (fg)"> or equals to f''g+fg''>0 but I ain't sure how that came about.

Thanks!
 
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cjaylee said:
f'<0, f''<0
g'<0, g''<0

They are still positive and concave, aren't they?
 
If f and g are both decreasing, then f'<0 and g'<0. Is that right? And if both the slopes are decreasing, then f''<0 and g''<0?
 
That does not mean that the slopes are decreasing. Suppose for example f = 1/x.
 
It says in the text that if f'>0, then f is increasing and if f'<0, then f is decreasing on the interval?
 
... and the text would be correct. However, f is not the slope, f is the function. f' is the slope and knowing that the slope is negative does definitely not tell you how it is changing.
 
Oh okay. Thanks.
 

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