Starting with Spivak - question

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    Spivak
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Discussion Overview

The discussion revolves around understanding the domain of the function f(x) = x² when applying a transformation to it, specifically f(x+1). Participants are examining the implications of shifting the function's input and how it affects the domain, as presented in Spivak's calculus book.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions why the domain for f(x+1) includes a restriction of -1 at the end, suggesting confusion over the transformation applied to the function.
  • Another participant clarifies that the domain of f(x+1) is indeed -18 ≤ x ≤ (π/3) - 1, which is derived from the original domain of f(x) and the transformation applied.
  • A participant expresses their misunderstanding by stating that they believe f(x+1) should extend the domain to -16 to π/3 + 1, indicating a misinterpretation of the function's transformation.
  • Further clarification is provided that the graph of y = f(x + 1) represents a leftward shift of the graph of y = f(x), maintaining that the domain must adjust accordingly.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correct interpretation of the domain for f(x+1). Some assert that the domain should be -18 ≤ x ≤ (π/3) - 1, while others propose a different interpretation, leading to confusion and a lack of consensus.

Contextual Notes

There is uncertainty regarding the application of transformations to the function and how they affect the domain. Participants have differing views on the implications of shifting the function's input, which remains unresolved.

robjow
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Hey, I'm just starting out with calculus and am giving the Spivak book a try per threads on here. In the introduction to functions chapter spivak defines a function f(x) =x^2. for all x such that -17 ≤ x ≤ π/3
Then says you should be able to check the following assertion about the function defined above:

f(x+1) = f(x) + 2x + 1 if -17 ≤ x ≤ (π/3) - 1

I don't understand why there a negative one is now needed at the end of the domain
 
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-18 ≤ x ≤ (π/3) - 1 is the domain of f(x+1)
-17 ≤ x ≤ (π/3) is the domain of f(x)
-17 ≤ x ≤ (π/3) - 1 is the domain the two share

edit:to correct error
 
Last edited:
thanks for your reply,

That doesn't make sense to me.

My incorrect thinking goes like this..

What is f(x+1) actually saying? To me its saying: apply the function f (which is to square) to the domain x (defined above) +1..making the domain of f(x+1) -16 to π/3 + 1
 
Shouldn't the domain of f(x+1) be -18 ≤ x ≤ (π/3) - 1??
 
robjow said:
That doesn't make sense to me.

My incorrect thinking goes like this..

What is f(x+1) actually saying? To me its saying: apply the function f (which is to square) to the domain x (defined above) +1..making the domain of f(x+1) -16 to π/3 + 1
The graph y = f(x + 1) is the graph of y = f(x) shifted (translated) one unit to the left. Since the domain of y = f(x) is -17 ≤ x ≤ π/3, the domain of y = f(x + 1) will be -18 ≤ x ≤ π/3 - 1. IOW, the original interval shifted one unit to the left.

Forget the weird domain for a moment. You know what the graph of y = f(x) = x2 looks like, right? The graph of y = g(x) = f(x + 1) is a parabola the opens up, and whose vertex is at (-1, 0). Every point on the shifted parabola is one unit to the left of its corresponding point on the graph of y = f(x).

robjow said:
Shouldn't the domain of f(x+1) be -18 ≤ x ≤ (π/3) - 1??
Yes, which is different from what you said above.
 

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