Let \(g(x)=\frac{2x^2-1}{x}\) and \(h(x)=2+\frac{1}{x^2}\), where \(x\neq0\).
a) Find \(g'(x)\), and use it to show that \(g\) is an antiderivative of \(h\).
b) Find equations of all asymptotes of the graph of \(g\).
c) Determine the intervals on which the graphs of \(g\) and \(h\) are concave up and concave down.
d) Explain why \(g\), \(h\), and \(h'\) have the same monotonic behavior on \((-\infty,0)\).
Hints
- Rewrite the rational expression before differentiating.
- Separate the linear part from the term that approaches zero to find the slant asymptote.
- Use the sign of each second derivative to determine concavity.
- Use the sign of each first derivative to determine monotonicity.
Solution
1. Rewrite \(g(x)=2x-\frac{1}{x}\). Then \(g'(x)=2+\frac{1}{x^2}=h(x)\), so \(g\) is an antiderivative of \(h\).
2. The function is undefined at \(x=0\), and its values become unbounded there, so \(x=0\) is a vertical asymptote. Since \(g(x)=2x-\frac{1}{x}\) and \(-\frac{1}{x}\to0\) as \(|x|\to\infty\), the slant asymptote is \(y=2x\).
3. For \(g\), \(g''(x)=h'(x)=-\frac{2}{x^3}\). This is positive when \(x<0\) and negative when \(x>0\), so \(g\) is concave up on \((-\infty,0)\) and concave down on \((0,\infty)\). For \(h\), \(h''(x)=\frac{6}{x^4}>0\), so \(h\) is concave up on both \((-\infty,0)\) and \((0,\infty)\).
4. On \((-\infty,0)\), \(g'(x)=2+\frac{1}{x^2}>0\), so \(g\) is strictly increasing. Also, \(h'(x)=-\frac{2}{x^3}>0\), so \(h\) is strictly increasing. Finally, \(h''(x)=\frac{6}{x^4}>0\), so \(h'\) is strictly increasing. All three are strictly increasing on that interval.
Answer
a) \(g'(x)=2+\frac{1}{x^2}=h(x)\), so \(g\) is an antiderivative of \(h\).
b) Vertical asymptote: \(x=0\). Slant asymptote: \(y=2x\).
c) \(g\) is concave up on \((-\infty,0)\) and concave down on \((0,\infty)\). The graph of \(h\) is concave up on \((-\infty,0)\cup(0,\infty)\).
d) \(g\), \(h\), and \(h'\) are all strictly increasing on \((-\infty,0)\).