Derivatives: Solving a Substitution Error on MathsIsFun

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    Error Substitution
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Discussion Overview

The discussion revolves around the interpretation of the function f(x) = x^2 and the calculation of f(x + dx). Participants are examining the mathematical expression and its implications in the context of derivatives, specifically addressing a perceived substitution error on a website.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether substituting x^2 into f(x + dx) should yield (x^2 + dx^2) instead of (x + dx)^2.
  • Another participant argues that f(x) = x^2 indicates that the function takes an input value and squares it, thus f(x + dx) correctly results in (x + dx)^2.
  • A later reply reiterates the explanation that the function is about squaring the input value, clarifying that f(x + dx) is not the same as x^2 + (dx)^2.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of the substitution in the function. There is no consensus reached on the initial participant's claim about the substitution error.

Contextual Notes

Some assumptions about the definitions of the function and the nature of the substitution are not fully explored, leading to differing interpretations of the mathematical expression.

kolleamm
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I'm learning about derivatives and on the website they put the value

x^2 into f(x + dx) and it became
(x + dx)^2

Shouldn't it be (x^2 + dx^2) ?

It's the last example

https://www.mathsisfun.com/calculus/derivatives-dy-dx.html

Thanks in advance!
 
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You're thinking of it wrong. The function f(x) = x^2 says, "take the input value and square it". So f(x+dx) means "take (x+dx) and square it", which is (x+dx)^2.
 
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phyzguy said:
You're thinking of it wrong. The function f(x) = x^2 says, "take the input value and square it". So f(x+dx) means "take (x+dx) and square it", which is (x+dx)^2.
That makes sense thank you
 
kolleamm said:
I'm learning about derivatives and on the website they put the value

x^2 into f(x + dx) and it became
(x + dx)^2

Shouldn't it be (x^2 + dx^2) ?
They aren't "putting the value x^2 into f(x + dx)" -- they are saying that ##f(x) = x^2## and are then calculating ##f(x + dx)## (or more likely, ##f(x + \Delta x)## ). As phyzguy noted, the given function squares its input value, so ##f(x + \Delta x) = (x + \Delta x)^2 \ne x^2 + (\Delta x)^2##.
 

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