Calculus Where did my textbook say clearly what it states at this point?

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The discussion revolves around a quote from the Spanish 6th edition of "Calculus" by Robert A. Adams, which states that the sign of the derivative indicates whether a function is increasing or decreasing. The original poster expresses curiosity about whether this concept was clearly explained earlier in the textbook. Participants clarify that the definition of the derivative and its implications for function behavior are foundational concepts that should be intuitive. They emphasize that understanding the derivative's sign is essential for determining function behavior without needing to rely solely on formal definitions. The conversation concludes with the affirmation that the textbook does provide clear explanations on this topic.
mcastillo356
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Hi, PF

I left reading for a while, and now I must revisit a quote from Spanish 6th edition of "Calculus", by Robert A. Adams. The quote is " 4.2 Extreme values problems (...) As we've seen, the sign of ##f'## shows if ##f## is increasing or decreasing".
Obvious, I shouldn't care and continue, but I'm kind of curious: does the book say it clearly previously, or must I read between lines what is written until now?

Best hopes, greetings
 
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The derivative is the rate of increase or decrease of the function value. A positive derivative implies an increasing function and a negative derivative implies a decreasing function. If the derivative is zero, the function is neither increasing nor decreasing.

The author must have covered that somewhere when he introduced the derivative.
 
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DEFINITION 3 The slope of a curve
The slope of a curve ##C## at a point ##P## is the slope of the tangent line to ##C## at ##P## if such tangent line exists. In particular, the slope of the graph of ##y=f(x)## at the point ##x_0## is

$$\displaystyle\lim_{h \to{0}}{\dfrac{f(x_0+h)-f(x_0)}{h}}$$

Well, here it is, I guess. Nevermind... It was just curiosity. The underlying true is clearly said at #2. Thanks.

On the road again, I hope.
 
mcastillo356 said:
DEFINITION 3 The slope of a curve
The slope of a curve ##C## at a point ##P## is the slope of the tangent line to ##C## at ##P## if such tangent line exists. In particular, the slope of the graph of ##y=f(x)## at the point ##x_0## is

$$\displaystyle\lim_{h \to{0}}{\dfrac{f(x_0+h)-f(x_0)}{h}}$$

Well, here it is, I guess. Nevermind... It was just curiosity. The underlying true is clearly said at #2. Thanks.

On the road again, I hope.
That's the formal mathematical definition of the derivative. That didn't come out of nowhere. That definition is intended to define formally something very important that already had a well understood intuitive meaning: the rate of change of a function at a point.

When you learn that definition, you should check that it makes sense and does indeed have the properties that you expect of the derivative. It should make sense as a formal way to make the derivative rigorous.

For example, "the sign of ##f'(x_0)## determines whether the function is increasing or decreasing at ##x_0##" is something we should already know from our elementary understanding of functions - it doesn't need knowledge of the formal definition to tell us that.
 
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PeroK said:
For example, "the sign of ##f'(x_0)## determines whether the function is increasing or decreasing at ##x_0##" is something we should already know from our elementary understanding of functions - it doesn't need knowledge of the formal definition to tell us that.
Definitely. Wrong, not accurate, my quote.
 
mcastillo356 said:
As we've seen, the sign of f′ shows if f is increasing or decreasing".
Obvious, I shouldn't care and continue, but I'm kind of curious: does the book say it clearly previously, or must I read between lines what is written until now?
As @PeroK pointed out, you don't really need the definition of the derivative to determine whether a function is increasing or decreasing, assuming you are able to draw a graph of the function or are able to compare function inputs vs. their outputs.

Once you have the derivative as a tool to use, you can determine the intervals on which a function is increasing/decreasing just by noting whether the derivative is positive or negative, respectively.

So the quoted text seems pretty clear to me.
 
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Capitulo 2, Teorema 12
Sea J un intervalo abierto, y sea I un intervalo que contiene a todos los puntos de J, y
posiblemente uno de sus extremos, o ambos. Sea f una función continua en I y diferencia-
ble en J.
(a) Si f'(x) > 0 para todo x perteneciente a J, entonces f es creciente en I.
(b) Si f'(x) < 0 para todo x perteneciente a J, entonces f es decreciente en I.
(c) Si f'(x) ≥ 0 para todo x perteneciente a J, entonces f es no decreciente en I.
(d) Si f'(x) ≤ 0 para todo x perteneciente a J, entonces f es no creciente en I.
 
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