Does a Non-Injective Differentiable Function Have a Zero Derivative Point?

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SUMMARY

The discussion centers on the application of the Mean Value Theorem (MVT) to non-injective differentiable functions. It establishes that if a function f(x) is differentiable on an interval and not injective, there exist points x1 and x2 within that interval such that f(x1) = f(x2). By applying the MVT to the interval [x1, x2], it is concluded that there must be at least one point c in (x1, x2) where the derivative f'(c) equals zero.

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If an interval is differentiable, but not injective, will there be a point where the derivative f'(x)=0 on that interval?

I'm not really sure how to approach this question. Help please?
 
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Use the mean value theorem. f f(x) is not injective, then there must exist x1 and x2 such that f(x1)= f(x2). Apply the mean value theorem to the interval [x1, x2].
 

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