So I understand how to find the inflection points for the graph of $f(x)$.
But basically, I have been shown a graph of an example function $f(x)$ and asked the state the inflection points of the graph. (I am just shown a curve… not given an actual function in terms of $x$)
Easy enough… I can see by looking where the concavity of the curve changes. But here is the tricky part. I am then asked to find the inflection points of the curves of $f'(x)$ and $f''(x)$.
From the graph of $f(x)$, I can draw in the values it cuts the $x$-axis, and whether it is positive/negative, but I don't understand how you can also comment on the inflection points from it?
i.e. $f(x)$ has both absoulute and local max values at $x = 2$ and $x = 6$, and local min at $x = 4$. Hence, $f'(x)$ cuts $x$ axis at $2, 4$ and $6$. I would imagine the local min/max would therefore be $3$ and $5$ (middle of $2$ & $4$, and $4$ & $6$ respectively)
The book gives answers a) $(3,5)$ Got it! b) $2, 4, 6$ c)$1, 7$
How do you find the point of inflection for $f'(x)$ and $f"(x)$ in this case?
Best Answer
An inflection point is where the second derivative is zero and changes sign. Thus, if the graph is of the second derivative then notice that the zeroes with sign change are at 1 and 7 on that graph.
If the graph is of the first derivative then the local min/max will be the inflection points since this is where if one were to look at the derivative of this graph it would be the second derivative that goes to zero at the extreme point as well as the sign change that is the criteria.