Homographic Functions A59Ebd
1. We are asked to determine the function rule (voorschrift) of a homographic function given certain conditions.
2. A homographic function has the general form:
$$f(x) = \frac{ax + b}{cx + d}$$
where $a$, $b$, $c$, and $d$ are constants and $cx + d \neq 0$.
3. Important properties:
- The zero (nulwaarde) is the value of $x$ for which $f(x) = 0$, so $ax + b = 0$.
- The vertical asymptote (pool) is where the denominator is zero, so $cx + d = 0$.
- The horizontal asymptote is the limit of $f(x)$ as $x \to \pm \infty$, which is $\frac{a}{c}$ if $c \neq 0$.
4. For the first problem (c):
- Zero at $x = -\frac{1}{3}$ means:
$$a\left(-\frac{1}{3}\right) + b = 0 \implies -\frac{a}{3} + b = 0 \implies b = \frac{a}{3}$$
- Horizontal asymptote at $y = 2$ means:
$$\lim_{x \to \infty} f(x) = \frac{a}{c} = 2 \implies a = 2c$$
- The graph passes through $P(-1, -8)$, so:
$$f(-1) = \frac{a(-1) + b}{c(-1) + d} = -8$$
Substitute $b = \frac{a}{3}$ and $a = 2c$:
$$\frac{-a + \frac{a}{3}}{-c + d} = -8 \implies \frac{-\frac{2a}{3}}{-c + d} = -8$$
Replace $a = 2c$:
$$\frac{-\frac{4c}{3}}{-c + d} = -8 \implies \frac{-\frac{4c}{3}}{-c + d} = -8$$
Multiply both sides by $-c + d$:
$$-\frac{4c}{3} = -8(-c + d) = 8c - 8d$$
Rearranged:
$$8c - 8d + \frac{4c}{3} = 0 \implies 8c + \frac{4c}{3} = 8d$$
Calculate left side:
$$8c = \frac{24c}{3}$$
So:
$$\frac{24c}{3} + \frac{4c}{3} = \frac{28c}{3} = 8d \implies d = \frac{28c}{24} = \frac{7c}{6}$$
5. We can choose $c = 6$ to clear denominators:
Then $a = 2c = 12$, $b = \frac{a}{3} = 4$, and $d = \frac{7c}{6} = 7$.
6. The function is:
$$f(x) = \frac{12x + 4}{6x + 7}$$
7. For the second problem (d):
- Vertical asymptote (pool) at $x = -\frac{1}{2}$ means:
$$c\left(-\frac{1}{2}\right) + d = 0 \implies -\frac{c}{2} + d = 0 \implies d = \frac{c}{2}$$
- Horizontal asymptote at $y = -5$ means:
$$\lim_{x \to \infty} f(x) = \frac{a}{c} = -5 \implies a = -5c$$
- The graph passes through $P(1, 2)$, so:
$$f(1) = \frac{a(1) + b}{c(1) + d} = 2$$
Substitute $a = -5c$ and $d = \frac{c}{2}$:
$$\frac{-5c + b}{c + \frac{c}{2}} = 2 \implies \frac{-5c + b}{\frac{3c}{2}} = 2$$
Multiply both sides by $\frac{3c}{2}$:
$$-5c + b = 2 \times \frac{3c}{2} = 3c$$
Solve for $b$:
$$b = 3c + 5c = 8c$$
8. Choose $c = 1$ for simplicity:
Then $a = -5$, $b = 8$, $d = \frac{1}{2}$.
9. The function is:
$$f(x) = \frac{-5x + 8}{x + \frac{1}{2}} = \frac{-5x + 8}{x + 0.5}$$
Final answers:
- (c) $$f(x) = \frac{12x + 4}{6x + 7}$$
- (d) $$f(x) = \frac{-5x + 8}{x + 0.5}$$