Math I Assignment 1
1. Solve inequalities:
I. Solve $|2x + 3| \leq |2x + 1|$:
- Square both sides: $(2x+3)^2 \leq (2x+1)^2$.
- Expand: $4x^2 + 12x + 9 \leq 4x^2 + 4x + 1$.
- Subtract $4x^2$: $12x + 9 \leq 4x + 1$.
- Simplify: $12x - 4x \leq 1 - 9 \Rightarrow 8x \leq -8$.
- Divide: $x \leq -1$.
- Check equality in absolute values region; solution is $x \leq -1$.
II. Solve $|x - 3| \leq 4$:
- Inequality means $-4 \leq x - 3 \leq 4$.
- Add 3: $-1 \leq x \leq 7$.
III. Solve $\frac{x^2 - 3x - 54}{x^2 - 5x - 14} \geq 0$:
- Factor numerator: $x^2 - 3x -54 = (x - 9)(x + 6)$.
- Factor denominator: $x^2 - 5x - 14 = (x - 7)(x + 2)$.
- Critical points: $-6, 7, -2, 9$.
- Test intervals:
- $(-\infty, -6)$: Numerator $+$, Denominator $+$, quotient $+$.
- $(-6, -2)$: Numerator $-$, Denominator $+$, quotient $-$.
- $(-2, 7)$: Numerator $-$, Denominator $-$, quotient $+$.
- $(7, 9)$: Numerator $-$, Denominator $+$, quotient $-$.
- $(9, \infty)$: Numerator $+$, Denominator $+$, quotient $+$.
- Exclude where denominator zero: $x \neq 7, -2$.
- Solution: $(-\infty, -6] \cup (-2, 7) \cup [9, \infty)$.
IV. Solve $|3x -18| > 6$:
- Divide by 3: $|x -6| > 2$.
- Which means $x -6 < -2$ or $x - 6 > 2$.
- So $x < 4$ or $x > 8$.
2. Domain and range:
I. $f(x) = \sqrt{25 - x^2}$:
- Domain: $25 - x^2 \geq 0 \Rightarrow -5 \leq x \leq 5$.
- Range: $f(x) \geq 0$; max at $x=0$ is 5, min 0.
- Range: $[0,5]$.
II. $f(x) = 3 + \frac{1}{2x - 5}$:
- Denominator $\neq 0 \Rightarrow 2x - 5 \neq 0 \Rightarrow x \neq \frac{5}{2}$.
- Domain: $\mathbb{R} \setminus \{2.5\}$.
- Range: $\mathbb{R} \setminus \{3\}$ (since $f(x) \neq 3$).
III. $f(x) = \sqrt{x^2 - 9}$:
- Inside square root $\geq 0$: $x^2 \geq 9$.
- Domain: $(-\infty, -3] \cup [3, \infty)$.
- Range: $[0, \infty)$.
IV. $f(x) = -1 + \frac{1}{x^2 + 25}$:
- $x^2 + 25 > 0$ for all real x.
- Domain: $\mathbb{R}$.
- Range: $\frac{1}{x^2 + 25} \in (0, \frac{1}{25}]$, so $f(x) \in (-1, -1 + \frac{1}{25}] = (-1, -\frac{24}{25}]$.
3. Determine if curve is function, domain, range:
I. Curve increasing from bottom left crossing $y=3$ near $x=4$:
- Assuming vertical line passes once, function.
- Domain: all $x$ shown in graph.
- Range: all $y$ values from bottom to max.
II. Step function with horizontal jumps at integers:
- Vertical lines indicate multiple $y$ for some $x$? No, jumps occur only at points, not violating vertical line test.
- It is a function.
- Domain: integers and segments covered.
- Range: values of steps.
4. Even/odd/neither:
V. $f(x)=\frac{x^3 + x -1}{x^2+1}$
- Compute $f(-x) = \frac{-x^3 - x -1}{x^2 +1}$.
- Not equal $f(x)$ nor $-f(x)$, so neither.
VI. $f(x) = \frac{x^3}{|x|} = x^2 \frac{x}{|x|} = x^2 \operatorname{sgn}(x)$
- $f(-x) = (-x)^2 \operatorname{sgn}(-x) = x^2 (-1) = -f(x)$
- Odd function.
VII. $f(x) = \frac{1}{x^4 + x^2 +5}$
- $f(-x) = f(x)$
- Even function.
VIII. $f(x) = x^2 - 6x + 9$
- $f(-x) = x^2 + 6x + 9 \neq \pm f(x)$
- Neither even nor odd.
5. Intervals increasing/decreasing:
I. $f(x) = 1 - 4x^5$
- $f'(x) = -20 x^4 \leq 0$
- Function decreasing everywhere except flat at $x=0$.
II. $f(x) = 4 - \frac{3}{\sqrt{x^2 +1}}$
- $f'(x) = \frac{3x}{(x^2+1)^{3/2}}$.
- Increasing where $x>0$, decreasing where $x<0$.
III. $f(x) = \cos x$ on $[0, 3\pi]$
- Derivative $f'(x) = -\sin x$.
- Increasing on $(\pi, 2\pi)$ where $\sin x < 0$.
- Decreasing on $(0, \pi)$ and $(2\pi, 3\pi)$.
IV. $f(x) = \frac{1}{x^3 +1}, x>0$
- Derivative: $f'(x) = -\frac{3x^2}{(x^3 +1)^2} <0$.
- Decreasing on $(0, \infty)$.
6. Bounded or not:
V. $f(x) = \frac{2}{1 + 5x^2}$
- Denominator $\geq 1$, function $\leq 2$ and $>0$.
- Bounded.
VI. $f(x) = \frac{1}{x+1} + \frac{1}{x^2 +2}, x \in (0, 2]$
- Near $x \to 0^+$, first term finite positive, second term finite.
- At $x=2$, finite.
- Function is bounded on domain.
Final answer contained above.