Concavity and Inflection Points

Concavity and Inflection Points
While the first derivative tells us about where a function is increasing and decreasing, the second derivative tells us about a functions Concavity. Concavity is the "U" shape of the graph (either up or down).
Concavity
Assume f′(x) is differentiable on the interval (a,b)
- If f′′(x)>0 then f(x) is concave up
- If f′′(x)<0 then f(x) is concave down
Inflection Points
An inflection point of a function f(x) is a point ,in its domain, at which there is a change in concavity in the graph of f(x)

Example: Concavity and Inflection Points
Determine the intervals of concavity and locate any inflection points for f(x)=ln(1+x2)
NOTE: There is an error in the video. From 4:35-4:50, the presenter mistakenly swaps which intervals are concave up and concave down. Otherwise, everything in the video and written solution is correct.
f′(x)=1+x22x
f′′(x)=(1+x2)22(1+x2)−2x(2x)
=(1+x2)22−2x2
f′′(x)=0 at x=±1

Concave down for (−∞,−1)∪(1,∞)
Concave up for (−1,1)
(−1,ln2) and (1,ln2) are inflection points