# Inflection Points

• Dec 21st 2013, 09:20 AM
turbozz
Inflection Points
So the question indicates: Write a statement/proof that ifwe have a point c with f′′(c) = 0, we can sometimes use the third derivative to determinewhether f has an inflection point at c, and of which type.

So, I understand how to write a statement and proof of how one can use the third derivative to determine if f has an inflection point (although only in certain cases). Now I'm not sure what the question 'and of which type' refers to. Is it referring to whether the curve goes concave up to concave down or goes concave down to concave up?
• Dec 21st 2013, 12:42 PM
SlipEternal
Re: Inflection Points
Yes, that is what it is referring to. It is similar to verifying maximums and minimums using the second derivative test.
• Dec 21st 2013, 02:19 PM
turbozz
Re: Inflection Points
So basically, if c is an inflection point, then if f'''(c)<0, the curve goes from concave up to concave down, and if f'''(x)>0 then the curve goes from concave down to concave up (just to get the basic idea, not meant to be completely formal)
• Dec 21st 2013, 02:21 PM
SlipEternal
Re: Inflection Points
Yes, that is the basic idea.
• Dec 21st 2013, 02:24 PM
turbozz
Re: Inflection Points
kk Thanks.
• Dec 23rd 2013, 07:35 AM
turbozz
Re: Inflection Points
Just had another quick question. Do you think it would be better to have two separate theorems, with one indicating whether or not the point is an inflection point, and the second determining its concavity? If I put it all into one theorem, then I end up repeating the statement 'then (c, f(c)) is an inflection point' both when f'''(x)>0 and f'''(x)<0.
• Dec 23rd 2013, 09:23 AM
turbozz
Re: Inflection Points
Nm, the proof works out better with one theorem.