Let \(f(x)=\frac{1}{2}x^2+1\), with domain \([0,\infty)\).
1) Find a formula for \(f^{-1}\) and state its domain.
2) Show algebraically that the graph of \(f\) does not intersect the line \(y=x\). Explain what this tells you about the locations of the graphs of \(f\) and \(f^{-1}\) relative to \(y=x\).
3) Find the slope of the tangent to the graph of \(f\) at \(x=2\). Use this result to find the slope of the tangent to the graph of \(f^{-1}\) at \(x=3\).
Hints
- Interchange the input and output, then solve while respecting the restricted domain.
- Examine the sign of \(f(x)-x\) only where \(f\) is defined.
- Inverse graphs are reflections across \(y=x\).
- At corresponding points, inverse-function derivative values are reciprocals.
Solution
1. From \(y=\frac{1}{2}x^2+1\), obtain \(x^2=2y-2\). Because the original domain requires \(x\ge0\), \(x=\sqrt{2y-2}\).
2. Thus, \(f^{-1}(x)=\sqrt{2x-2}\). The range of \(f\) is \([1,\infty)\), so the inverse domain is \([1,\infty)\).
3. For every \(x\) in the domain \([0,\infty)\), \(f(x)-x=\frac{1}{2}x^2-x+1=\frac{1}{2}(x-1)^2+\frac{1}{2}>0\). Therefore, the graph of \(f\) lies above \(y=x\) and never intersects it.
4. Since inverse graphs are reflections across \(y=x\), the graph of \(f^{-1}\) lies below \(y=x\). The two graphs lie on opposite sides of the line and do not intersect.
5. Since \(f^{\prime}(x)=x\), \(f^{\prime}(2)=2\), and \(f(2)=3\). Therefore, \(\left(f^{-1}\right)^{\prime}(3)=\frac{1}{f^{\prime}(2)}=\frac{1}{2}\).
Answer
1) \(f^{-1}(x)=\sqrt{2x-2}\), with domain \([1,\infty)\)
2) The graph of \(f\) lies above \(y=x\), and the graph of \(f^{-1}\) lies below \(y=x\); they do not intersect.
3) The slopes are \(2\) for \(f\) at \(x=2\) and \(\frac{1}{2}\) for \(f^{-1}\) at \(x=3\).