Determine where the function is concave upward and downwardDetermine where the function is concave upward and downward, and list all the x- value(s) of the point(s) of inflection:...
Determine where the function is concave upward and downward
Determine where the function is concave upward and downward, and list all the x- value(s) of the point(s) of inflection: f(x)=3x^3+(1)⋅x^2+1x−9. (Do not concatenate your answers. i.e., if the intervals where the function is concave downward are, e.g., (0,1) and (1,3), don't write (0,3); list the intervals separately. For ∞, type inf , and for −∞, type - inf .) The function is concave downward on the interval(s) . The function is concave upward on the interval(s) . The x- value(s) of the point(s) of inflection is (are) . (If there are none, then enter none .)]
1 Answer
embizze | High School Teacher | (Level 2) Educator Emeritus
Find the inflection points (if any) and the intervals where the function is concave up and concave down for `f(x)=3x^3+x^2+x-9` :
A function is concave up on an interval if the second derivative is positive on the interval; concave down if the second derivative is negative.
`f(x)=3x^3+x^2+x-9`
`f'(x)=9x^2+2x+1`
`f''(x)=18x+2`
`f''(x)=0 ==>x=-1/9`
We try test points on the intervals `(-oo,-2/9),(-2/9,oo)` :
`f''(-1)=-16<0` so the function is concave down on `(-oo,-2/9)`
`f''(0)=2>0` so the function is concave up on `(-2/9,oo)`
Since the concavity changes from down to up at `x=-2/9` and the second derivative exists there` `, `(-2/9,-2237/243)` is the only inflection point.
The graph: