009A Sample Final 1, Problem 10

Consider the following continuous function:

$f(x)=x^{1/3}(x-8)$

defined on the closed, bounded interval $[-8,8]$.

(a) Find all the critical points for $f(x)$.

(b) Determine the absolute maximum and absolute minimum values for $f(x)$ on the interval $[-8,8]$.

Foundations:
Recall:
1. To find the critical points for $f(x),$ we set $f'(x)=0$ and solve for $x.$
Also, we include the values of $x$ where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f'(x)} is undefined.
2. To find the absolute maximum and minimum of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)} on an interval Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle [a,b],}
we need to compare the Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y} values of our critical points with Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(a)} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(b).}

Solution:

(a)

Step 1:
To find the critical points, first we need to find $f'(x).$
Using the Product Rule, we have
${\begin{array}{rcl}\displaystyle {f'(x)}&=&\displaystyle {{\frac {1}{3}}x^{-{\frac {2}{3}}}(x-8)+x^{\frac {1}{3}}}\\&&\\&=&\displaystyle {{\frac {x-8}{3x^{\frac {2}{3}}}}+x^{\frac {1}{3}}.}\\\end{array}}$

Step 2:
Notice $f'(x)$ is undefined when $x=0.$
Now, we need to set $f'(x)=0.$
So, we get
$-x^{\frac {1}{3}}\,=\,{\frac {x-8}{3x^{\frac {2}{3}}}}.$
We cross multiply to get $-3x=x-8.$
Solving, we get Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle x=2.}
Thus, the critical points for $f(x)$ are $(0,0)$ and $(2,2^{\frac {1}{3}}(-6)).$

(b)

Step 1:
We need to compare the values of $f(x)$  at the critical points and at the endpoints of the interval.
Using the equation given, we have $f(-8)=32$  and $f(8)=0.$

Step 2:
Comparing the values in Step 1 with the critical points in (a), the absolute maximum value for $f(x)$  is $32$
and the absolute minimum value for $f(x)$  is $2^{\frac {1}{3}}(-6).$

Final Answer:
(a)  $(0,0)$ and $(2,2^{\frac {1}{3}}(-6))$
(b)  The absolute minimum value for $f(x)$ is $2^{\frac {1}{3}}(-6).$

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