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MathDork00
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Homework Statement
If f'x = 0 [tex]\forall[/tex] x [tex]\in[/tex] (a,b), show that f is constant there.
I've gotten a final result, but I'm not entirely sure if I actually showed what I intended to show...
Homework Equations
The Attempt at a Solution
Fix a point c [tex]\in[/tex] (a,b) and choose a point x [tex]\in[/tex] (a,b) different from c
From definition of differentiable:
f'(x) = lim (x[tex]{\rightarrow}[/tex]c) (f(x)-f(c))/ x - c
From problem, f'(x) = 0 = lim (x[tex]{\rightarrow}[/tex]c) (f(x)-f(c))/ x - c
From sums/products of limits,
lim (x[tex]{\rightarrow}[/tex]c) (f(x)-f(c))/ x - c = lim (x[tex]{\rightarrow}[/tex]c) f(x)/(x-c) - f(c)/(x-c)
implies 1/(x-c)lim (x[tex]{\rightarrow}[/tex]c) f(x) - 1/(x-c) lim (x[tex]{\rightarrow}[/tex]c) f(c) = 0
implies lim (x[tex]{\rightarrow}[/tex]c) f(x) = lim (x[tex]{\rightarrow}[/tex]c) f(c)
implies lim (x[tex]{\rightarrow}[/tex]c) f(x) = f(c)
This implies that the function will have the same value as f(c) when evaluated anywhere inside (a,b) and is therefore constant.
Sorry if my typing is messy, but I am not too proficient with LaTex! :-(