Formal Limits
1. The problem requires finding $\delta$ for each limit statement given $f(x)$, $x_0$, $L = \lim_{x \to x_0} f(x)$, and $\varepsilon$. We want $\delta > 0$ s.t. if $0 < |x - x_0| < \delta$ then $|f(x) - L| < \varepsilon$.\\
2. For exercise 31: $f(x) = 3 - 2x$, $x_0 = 3$, $\varepsilon = 0.02$.
- Limit: $L = 3 - 2(3) = 3 - 6 = -3$.
- Solve $|f(x) - L| < \varepsilon$:
$$|3 - 2x + 3| < 0.02 \implies |-2x + 6| < 0.02$$
$$2|x - 3| < 0.02 \implies |x - 3| < \frac{0.02}{2} = 0.01$$
So $\delta = 0.01$.
3. Exercise 32: $f(x) = -3x - 2$, $x_0 = -1$, $\varepsilon = 0.03$.
- Limit: $L = -3(-1) - 2 = 3 - 2 = 1$
- $|-3x - 2 - 1| < 0.03 \implies |-3x - 3| < 0.03$
- $3|x + 1| < 0.03 \implies |x + 1| < 0.01$
- $\delta = 0.01$.
4. Exercise 33: $f(x) = \frac{x^2 - 4}{x-2}$ at $x_0 = 2$, $\varepsilon = 0.05$.
- Simplify: $f(x) = \frac{(x-2)(x+2)}{x-2} = x + 2$, $x \ne 2$.
- Limit: $L = 2 + 2 = 4$.
- $|f(x) - 4| < 0.05 \implies |x + 2 - 4| < 0.05 \implies |x - 2| < 0.05$.
- So $\delta = 0.05$.
5. Exercise 34: $f(x) = \frac{x^2 + 6x + 5}{x + 5}$, $x_0 = -5$, $\varepsilon = 0.05$.
- Factor numerator: $(x + 1)(x + 5)$.
- Simplify for $x \ne -5$: $f(x) = x + 1$.
- Limit: $L = -5 + 1 = -4$.
- $|f(x) - L| < \varepsilon \implies |x + 1 + 4| < 0.05 \implies |x + 5| < 0.05$.
- $\delta = 0.05$.
6. Exercise 35: $f(x) = \sqrt{1 - 5x}$, $x_0 = -3$, $\varepsilon = 0.5$.
- Limit: $L = \sqrt{1 - 5(-3)} = \sqrt{1 + 15} = \sqrt{16} = 4$.
- Solve $|\sqrt{1-5x} - 4| < 0.5$.
- Square inequality bounds:
- Lower bound: $4 - 0.5 =3.5$, Upper bound: $4 + 0.5 = 4.5$.
- So $3.5 < \sqrt{1-5x} < 4.5$.
- Square both:
- $3.5^2 < 1 - 5x < 4.5^2$.
- $12.25 < 1 - 5x < 20.25$.
- Solve inequalities:
- Left: $1 - 5x > 12.25 \implies -5x > 11.25 \implies x < -\frac{11.25}{5} = -2.25$.
- Right: $1 - 5x < 20.25 \implies -5x < 19.25 \implies x > -\frac{19.25}{5} = -3.85$.
- So $-3.85 < x < -2.25$.
- Since $x_0 = -3$, distance to endpoints: min$(-3 + 3.85, -2.25 + 3) = \min(0.85, 0.75) = 0.75$.
- $\delta = 0.75$.
7. Exercise 36: $f(x) = \frac{4}{x}$, $x_0 = 2$, $\varepsilon = 0.4$.
- Limit: $L = \frac{4}{2} = 2$.
- Solve $|\frac{4}{x} - 2| < 0.4$.
- $|\frac{4 - 2x}{x}| < 0.4$.
- To avoid complexity, impose $x$ near 2, e.g. $|x - 2| < 1$ so $x \in (1, 3)$.
- For $x \in (1,3)$, $|x| \ge 1$.
- Multiply inequality by $|x| \ge 1$:
$|4 - 2x| < 0.4|x|$ and since $|x| < 3$, $|4 - 2x| < 1.2$.
- $|4 - 2x| = 2|2 - x| < 1.2 \implies |x - 2| < 0.6$.
- So $\delta = 0.6$.
Final answers summarized:
- 31: $\delta=0.01$
- 32: $\delta=0.01$
- 33: $\delta=0.05$
- 34: $\delta=0.05$
- 35: $\delta=0.75$
- 36: $\delta=0.6$.
This solves the first 6 exercises on formal limits with given $\varepsilon$.