Phil S.

asked • 09/20/16

True or false about a statement considering continuity and the derivative.

If f'(a) is defined, then f(x) is continuous at x=a.
 
True or False? I'm not exactly sure what this means. Isn't f'(a) the tangent slope? and since there can only be one it can't be continuous so this statement is invalid all together?
Please help me, I still don't understand these concepts very well...

2 Answers By Expert Tutors

By:

Mark M.

tutor
By definition, f'(2) = limh→0[(f(2+h)-f(2))/h]. 
 
For the given function, limh→0- [(f(2+h)-f(2))/h] = 5, but                      
limh→0+ [(f(2+h) - f(2))/h] ≠ 5.  Since the one-sided limits are not the same, f'(2) does not exist.  
 
Mark M (Bayport, NY)
 
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09/20/16

Steven W.

tutor
But it should exist, shouldn't it?  There in only one definition of f'(x) at x = 2, since so f'(2) is defined for that point.
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09/20/16

Steven W.

tutor
Even though that well defined value is not reached coming from above.
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09/20/16

Steven W.

tutor
Never mind.  I see my error.  You are correct.
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09/20/16

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