Answer
The function \( f(x) = \frac{x^2}{x-5} \) has critical numbers at \( x=0 \) and \( x=10 \). It is increasing on \( (-\infty,0) \) and \( (10,\infty) \), and decreasing on \( (0,5) \) and \( (5,10) \). The function has a local maximum at \( x=0 \) and a local minimum at \( x=10 \).
Solution
We begin with
\[
f(x)=\frac{x^2}{x-5},
\]
with domain all real numbers except \( x=5 \).
1. **Finding the critical numbers**
a. Find \( f'(x) \) using the quotient rule. Recall that if
\[
f(x)=\frac{g(x)}{h(x)},
\]
then
\[
f'(x)=\frac{g'(x)h(x)-g(x)h'(x)}{[h(x)]^2}.
\]
b. Here, \( g(x)=x^2 \) and \( h(x)=x-5 \). Then,
\[
g'(x)=2x \quad \text{and} \quad h'(x)=1.
\]
c. Applying the quotient rule, we have
\[
f'(x)=\frac{(2x)(x-5)-x^2(1)}{(x-5)^2} = \frac{2x(x-5)- x^2}{(x-5)^2}.
\]
d. Expand the numerator:
\[
2x(x-5)=2x^2-10x,
\]
so
\[
f'(x)=\frac{2x^2-10x-x^2}{(x-5)^2}=\frac{x^2-10x}{(x-5)^2}.
\]
e. Factor the numerator:
\[
x^2-10x=x(x-10).
\]
Thus,
\[
f'(x)=\frac{x(x-10)}{(x-5)^2}.
\]
f. Critical numbers occur when \( f'(x)=0 \) or is undefined (provided the point is in the domain). The numerator is zero when
\[
x(x-10)=0,
\]
so \( x=0 \) or \( x=10 \). The derivative is undefined when the denominator is zero, i.e. when
\[
(x-5)^2=0 \quad\Rightarrow\quad x=5.
\]
However, since \( f(x) \) is not defined at \( x=5 \) (vertical asymptote), we do not consider \( x=5 \) as a critical number.
g. Therefore, the critical numbers are
\[
x=0 \quad \text{and} \quad x=10.
\]
2. **Determining the intervals on which \( f(x) \) is increasing**
Since
\[
f'(x)=\frac{x(x-10)}{(x-5)^2},
\]
and \( (x-5)^2 \) is always positive for \( x\neq5 \), the sign of \( f'(x) \) is determined by the numerator \( x(x-10) \).
a. Identify the zeros of the numerator:
\[
x=0 \quad \text{and} \quad x=10.
\]
b. Test intervals determined by these values (and taking care of the vertical asymptote at \( x=5 \)):
• For \( x<0 \) (for example, \( x=-1 \)):
\( (-1)\cdot((-1)-10) = (-1)\cdot(-11) = 11 > 0 \).
Thus, \( f'(x)>0 \) and \( f(x) \) is increasing on \( (-\infty,0) \).
• For \( 010 \) (for example, \( x=11 \)):
\( 11\cdot(11-10)=11\cdot(1)=11 > 0 \).
Thus, \( f(x) \) is increasing on \( (10,\infty) \).
c. Therefore, the function is increasing on
\[
(-\infty,0) \quad \text{and} \quad (10,\infty).
\]
3. **Local extrema**
a. At \( x=0 \):
- For \( x<0 \), \( f'(x)>0 \) (increasing).
- For \( 010 \), \( f'(x)>0 \) (increasing).
Thus, \( f(x) \) has a local minimum at \( x=10 \).
The function value is
\[
f(10)=\frac{10^2}{10-5}=\frac{100}{5}=20.
\]
4. **Answering the Multiple-Choice Statement**
The statements provided are:
- A. The function has a local minimum at \( x=0 \) and a local maximum at \(\dots\)
- B. The function has a local minimum at \( x=10 \) and a local maximum at \( x=0 \).
- C. The function has no local extrema.
From our analysis, the correct statement is:
\[
\textbf{B. The function has a local minimum at } x=10 \textbf{ and a local maximum at } x=0.
\]
5. **Summary**
- The critical numbers of \( f(x)=\frac{x^2}{x-5} \) are \( x=0 \) and \( x=10 \).
- \( f(x) \) is increasing on \( (-\infty,0) \) and \( (10,\infty) \).
- \( f(x) \) is decreasing on \( (0,5) \) and \( (5,10) \).
- \( f \) has a local maximum at \( x=0 \) (with \( f(0)=0 \)) and a local minimum at \( x=10 \) (with \( f(10)=20 \)).
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