Inverse Functions
1. Problem (a): Find the inverse function $f^{-1}$ of the function $f(x) = -3 + \sqrt{3 + 2x}$ and determine the domain and range of $f^{-1}$.
2. To find the inverse, set $y = f(x) = -3 + \sqrt{3 + 2x}$.
3. Solve for $x$ in terms of $y$:
$$y = -3 + \sqrt{3 + 2x} \implies y + 3 = \sqrt{3 + 2x}$$
4. Square both sides to eliminate the square root:
$$ (y + 3)^2 = 3 + 2x $$
5. Isolate $x$:
$$ 2x = (y + 3)^2 - 3 \implies x = \frac{(y + 3)^2 - 3}{2} $$
6. The inverse function is therefore:
$$ f^{-1}(y) = \frac{(y + 3)^2 - 3}{2} $$
7. Now find the domain and range of $f^{-1}$. Since $f^{-1}$ swaps the domain and range of $f$, first find the domain and range of $f$.
8. The domain of $f$ is all $x$ values for which the expression under the square root is non-negative:
$$3 + 2x \geq 0 \implies x \geq -\frac{3}{2}$$
9. For $x \geq -\frac{3}{2}$, compute the range of $f(x)$:
$$ f(x) = -3 + \sqrt{3 + 2x} \geq -3 + 0 = -3 $$
As $x$ grows large, $f(x)$ increases without bound, so the range is:
$$ \mathrm{Range}(f) = [-3, \infty) $$
10. Therefore, the domain of $f^{-1}$ is $[-3, \infty)$ and the range of $f^{-1}$ is $[-\frac{3}{2}, \infty)$.
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11. Problem (b): Consider $f$ satisfying $[f(x)]^5 + f(x) = \ln x$ for $x > 0$. Show that $f^{-1}$ exists and evaluate $[f(1)]^5 - [f^{-1}(1)]^5$.
12. To show $f^{-1}$ exists, we need to prove $f$ is one-to-one and onto its range. Define $g(t) = t^5 + t$.
13. Note $g'(t) = 5t^4 + 1 > 0$ for all real $t$, so $g(t)$ is strictly increasing and hence one-to-one.
14. The equation $g(f(x)) = \ln x$ implies $f(x) = g^{-1}(\ln x)$, so $f$ is the composition of an increasing function $g^{-1}$ and an increasing function $\ln x$ (for $x>0$), hence $f$ is strictly increasing and injective.
15. Since $f$ is strictly increasing and defined on $(0, \infty)$, its inverse $f^{-1}$ exists.
16. Evaluate the expression:
$$ [f(1)]^5 - [f^{-1}(1)]^5 $$
17. Given $x=1$, we have:
$$ [f(1)]^5 + f(1) = \ln 1 = 0 $$
Let $a = f(1)$. Then:
$$ a^5 + a = 0 \implies a(a^4 + 1) = 0 $$
18. Since $a^4 + 1 > 0$, it follows $a=0$. Hence:
$$ f(1) = 0 $$
19. Next, find $f^{-1}(1)$, i.e., find $x$ such that:
$$ f(x) = 1 $$
20. Using the defining relation:
$$ [f(x)]^5 + f(x) = \ln x $$
Substitute $f(x) = 1$:
$$1^5 + 1 = \ln x \implies 2 = \ln x \implies x = e^{2}$$
21. Therefore:
$$ f^{-1}(1) = e^{2} $$
22. Evaluate the expression:
$$ [f(1)]^{5} - [f^{-1}(1)]^5 = 0^5 - (e^{2})^{5} = - e^{10} $$
Final answers:
- (a) $f^{-1}(y) = \frac{(y+3)^2 -3}{2}$ with domain $[-3, \infty)$ and range $[-\frac{3}{2}, \infty)$
- (b) Inverse $f^{-1}$ exists and
$$ [f(1)]^{5} - [f^{-1}(1)]^{5} = - e^{10} $$