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  $[-8,8]$.

ExpandFoundations:
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  $f'(x)$  is undefined.
2. To find the absolute maximum and minimum of  $f(x)$  on an interval  $[a,b],$
we need to compare the  $y$  values of our critical points with  $f(a)$  and  $f(b).$

Solution:

(a)

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

ExpandStep 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  $x=2.$
Thus, the critical points for  $f(x)$  are  $(0,0)$  and  $(2,2^{\frac {1}{3}}(-6)).$

(b)

ExpandStep 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  $f(8)=0.$

ExpandStep 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  $2^{\frac {1}{3}}(-6).$

ExpandFinal Answer:
(a)    $(0,0)$  and  $(2,2^{\frac {1}{3}}(-6))$
(b)    The absolute maximum value for  $f(x)$  is  $32$  and the absolute minimum value for  $f(x)$  is  $2^{\frac {1}{3}}(-6).$

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