118 STEP 4. Review the Knowledge You Need to Score High
Step 4: Set up a table.
INTERVALS (−∞,2) x= 2 (2,∞)
Test Point 0 5
f′′(x) − 0 +
f(x) concave point of concave
downward inflection upward
Since f(x) has change of concavity atx=2, the point (2, f(2)) is a point of
inflection. f(2)=(2)^3 −6(2)^2 +12(2)− 8 =0.
Step 5: Write a conclusion.
Thus,f(x) is concave downward on (−∞, 2), concave upward on (2,∞) andf(x)
has a point of inflection at (2, 0). (See Figure 7.2-23.)
[–1,5] by [–5,5]
Figure 7.2-23
Example 4
Find the points of inflection of f(x)=(x−1)^2 /^3 and determine the intervals where the
functionfis concave upward and where it is concave downward.
Step 1: Find f′(x) andf′′(x).
f′(x)=
2
3
(x−1)−^1 /^3 =
2
3(x−1)^1 /^3
f′′(x)=−
2
9
(x−1)−^4 /^3 =
− 2
9(x−1)^4 /^3
Step 2: Find all values ofxwhere f′′(x)=0orf′′(x) is undefined.
Note that f′′(x)/=0 and that f′′(1) is undefined.
Step 3: Determine intervals.
The intervals are (−∞, 1), and (1,∞).