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Homework Statement
There is a part of Spivak's proof of L'Hospital's Rule that I don't really see justified. It is Theorem 11-9 on p.204 of Calculus, 4th edition.
I won't reproduce the statement of L'Hospital's Rule here, except to say that it is stated for the case [itex]\lim_{x\rightarrow a}f(x)=0[/itex] and [itex]\lim_{x\rightarrow a}g(x)=0[/itex] only (i.e. indeterminate form 0/0).
Spivak first explains two assumptions implicit in the hypothesis that [itex]\lim_{x\rightarrow a}\frac{f'(x)}{g'(x)}[/itex] exists:
1. there is an interval [itex](a-\delta, a+\delta)[/itex] on which [itex]f'(x)[/itex] and [itex]g'(x)[/itex] exist for all x in the interval, except possibly at [itex]a[/itex].
2. [itex]g'(x)\neq 0[/itex] in this interval, again except possibly at [itex]a[/itex].
He notes that [itex]f[/itex] and [itex]g[/itex] are not assumed to be defined at [itex]a[/itex].
***He then defines [itex]f(a)=g(a)=0[/itex], making [itex]f[/itex] and [itex]g[/itex] continuous at [itex]a[/itex].***
Having done so, he uses the differentiability of [itex]f[/itex] and [itex]g[/itex] on [itex](a, a+\delta)[/itex] to find [itex]x[/itex] so that [itex]f[/itex] and [itex]g[/itex] are continuous on [itex][a,x][/itex] and differentiable on [itex](a,x)[/itex]. The conditions satisfied, he applies the Mean Value Theorem and the Cauchy Mean Value Theorem to [itex]f[/itex] and [itex]g[/itex], and the rest of the proof is fairly straightforward.
My question, regarding the section marked by ***: why is he allowed to define [itex]f(a)=g(a)=0[/itex]? The theorem should hold even if [itex]f[/itex] and [itex]g[/itex] are not continuous at [itex]a[/itex]; Rudin provides such a proof in Principles, though I'm having trouble following it as of now. Spivak appears to have proved a weaker result for no good reason, unless this drastically simplifies the proof, and the full result is significantly harder to follow; but then he should at least mention the oversight.