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cra18
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I am currently reading Calculus Made Easy by S. P. Thompson, and the author's idea of what it means for a variable to "vary" seems fundamentally different from my own, so I was hoping someone could help me correct my understanding. Here is the excerpt I'm having trouble with:
Those [quantities] which we consider as capable of growing, or (as mathematicians say) of "varying," we denote by letters from the end of the alphabet, such as [itex]x, y, z, u, v, w, [/itex] or sometimes [itex] t [/itex] . . . Suppose we have two variables that depend on each other. An alteration in one will bring about an alteration in the other, because of this dependence. Let us call one of the variables [itex] x [/itex], and the other that depends on it [itex] y [/itex] . . . Suppose we make [itex] x [/itex] to vary, that is to say, we either alter it or imagine it to be altered, by adding to it a bit which we call [itex] \mathrm{d}x [/itex]. We are thus causing [itex] x [/itex] to become [itex] x + \mathrm{d}x [/itex]. Then, because [itex] x [/itex] has been altered, [itex] y [/itex] will have altered also, and will have become [itex] y + \mathrm{d}y [/itex].
My previous understanding is: a variable is an unspecified element of some set. So if I say that [itex] x [/itex] is a real number, this means that [itex] x [/itex] is an unspecified element of [itex] \mathbb{R} [/itex]. The fact that there is more than one element in [itex] \mathbb{R} [/itex] is the reason why [itex] x [/itex] is capable of "varying" --- i.e., it can potentially take anyone of the values in [itex] \mathbb{R} [/itex].
But [itex] x [/itex] varying to become [itex] x + \mathrm{d}x [/itex] doesn't make sense to me, [itex] x [/itex] being just a placeholder for an element of some set. I agree that the quantity [itex] x + \mathrm{d}x [/itex] is probably also an element of the same set that [itex] x [/itex] ranges over, but it seems more like a new variable than the result of simple variation. That is to say, it seems more appropriate to call [itex] x + \mathrm{d}x [/itex] the output of some underlying iterating function like [itex] g(x) = x + \mathrm{d}x [/itex], instead of a new value of the original [itex] x [/itex], in which case, if I define [itex] y = f(x) [/itex], then
[tex]
\begin{equation}
y + \mathrm{d}y = f(g(x)) = f(x + \mathrm{d}x).
\end{equation}
[/tex]
would be the corresponding definition. Could someone explain if the above is the correct way to think about "varying" as the author describes it, and whether my concept of a variable is correct?
Those [quantities] which we consider as capable of growing, or (as mathematicians say) of "varying," we denote by letters from the end of the alphabet, such as [itex]x, y, z, u, v, w, [/itex] or sometimes [itex] t [/itex] . . . Suppose we have two variables that depend on each other. An alteration in one will bring about an alteration in the other, because of this dependence. Let us call one of the variables [itex] x [/itex], and the other that depends on it [itex] y [/itex] . . . Suppose we make [itex] x [/itex] to vary, that is to say, we either alter it or imagine it to be altered, by adding to it a bit which we call [itex] \mathrm{d}x [/itex]. We are thus causing [itex] x [/itex] to become [itex] x + \mathrm{d}x [/itex]. Then, because [itex] x [/itex] has been altered, [itex] y [/itex] will have altered also, and will have become [itex] y + \mathrm{d}y [/itex].
My previous understanding is: a variable is an unspecified element of some set. So if I say that [itex] x [/itex] is a real number, this means that [itex] x [/itex] is an unspecified element of [itex] \mathbb{R} [/itex]. The fact that there is more than one element in [itex] \mathbb{R} [/itex] is the reason why [itex] x [/itex] is capable of "varying" --- i.e., it can potentially take anyone of the values in [itex] \mathbb{R} [/itex].
But [itex] x [/itex] varying to become [itex] x + \mathrm{d}x [/itex] doesn't make sense to me, [itex] x [/itex] being just a placeholder for an element of some set. I agree that the quantity [itex] x + \mathrm{d}x [/itex] is probably also an element of the same set that [itex] x [/itex] ranges over, but it seems more like a new variable than the result of simple variation. That is to say, it seems more appropriate to call [itex] x + \mathrm{d}x [/itex] the output of some underlying iterating function like [itex] g(x) = x + \mathrm{d}x [/itex], instead of a new value of the original [itex] x [/itex], in which case, if I define [itex] y = f(x) [/itex], then
[tex]
\begin{equation}
y + \mathrm{d}y = f(g(x)) = f(x + \mathrm{d}x).
\end{equation}
[/tex]
would be the corresponding definition. Could someone explain if the above is the correct way to think about "varying" as the author describes it, and whether my concept of a variable is correct?
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