TD 4

Index

Function Analysis

Exercise 1

$f(x)$ Domain Asymptotes Limits at Boundaries Axes Intercepts Increasing-Decreasing Intervals Concave-Convex Intervals Maximum-Minimum Points Inflection Points Plot
$$x^4-18x^2+56$$ $$\begin{aligned} y &= f(x) \\ \mathbf{X} &\longrightarrow \mathbf{Y} \\ \mathbf{X} &= (-\infty, +\infty) \\ \mathbf{X} &= \mathbb{R} \\ \end{aligned}$$
  • Given $x^*$ a finite boundary of $f(x)$,
    a vertical asymptote is present at
    $x_a=x^*$ when $\lim_{x \to x*}f(x) = \infty$
  • A horizontal asymptote is present at
    $y_a$ when $\lim_{x \to \infty} = y_a \not= \infty$
$$\begin{aligned} x_a &\in \{\} \text{ because } x^* \in \{\} \\ && \\ y_a &\in \{\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$
$$\begin{aligned} &\lim_{x \to -\infty} x^4-18x^2+56 \\ \sim &\lim_{x \to -\infty} x^4 \\ = &+\infty \\ && \\ &\lim_{x \to +\infty} x^4-18x^2+56 \\ \sim &\lim_{x \to +\infty} x^4 \\ = & +\infty \\ \end{aligned}$$ $$\begin{aligned} f(x) &= (x^2-4)(x^2-14) \\ &= (x+\sqrt{14})(x-\sqrt{14})(x+2)(x-2) \\ x_0 &\in \{-\sqrt{14}, -2, 2, \sqrt{14}\} \\ &\approx \{-3.74, -2, 2, 3.74\} \\ && \\ y_0 &= f(0) = 56 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= 4x^3-36x \\ &= 4x(x^2-9) \\ &= 4x(x-3)(x+3) \\ x'_0 &\in \{-3, 0, 3\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) < 0, && x &\in (-\infty,-3) && (-)\\ f'(x) > 0, && x &\in (-3,0) && (+) \\ f'(x) < 0, && x &\in (0,3) && (-) \\ f'(x) > 0, && x &\in (3, +\infty) && (+) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= 12x^2 - 36\\ &= 12(x^2-3) \\ &= 12(x-\sqrt{3})(x+\sqrt{3}) \\ x''_0 &\in \{-\sqrt{3}, \sqrt{3}\} \\ &\approx \{-1.73, 1.73\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) > 0, && x &\in (-\infty,-\sqrt{3}) && (\cup)\\ f''(x) < 0, && x &\in (-\sqrt{3},\sqrt{3}) && (\cap) \\ f''(x) > 0, && x &\in (\sqrt{3}, +\infty) && (\cup) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\in \{-3, 0, 3\} && \\ f''(-3) &= 12(-3)^2 - 36 > 0 &\Rightarrow min \\ f''( 0) &= 12( 0)^2 - 36 < 0 &\Rightarrow max \\ f''( 3) &= 12( 3)^2 - 36 > 0 &\Rightarrow min \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\in \{-\sqrt{3}, \sqrt{3}\} \\ \end{aligned}$$
figures/td4-1a.png
$$\frac{2x-x^2}{x^2-2x+1}$$ $$\begin{aligned} f(x) &= \frac{g(x)}{h(x)} \\ h(x) &= x^2-2x+1 \\ &= (x-1)^2 \\ x^* &\in \{1\} \\ && \\ \mathbf{X} &= (-\infty,1) \cup (1,+\infty) \\ \mathbf{X} &= \mathbb{R} \backslash \{1\} \\ \end{aligned}$$ $$\begin{aligned} x^* &\in \{1\} \\ &\lim_{x \to 1} \frac{2x-x^2}{x^2-2x+1} = \frac{1}{0} \to \infty \\ && \\ x_a &\in \{1\} \\ && \\ y_a &\in \{-1\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\infty} \frac{2x-x^2}{x^2-2x+1} \\ = &\lim_{x \to -\infty} \frac{\frac{2}{x}-1}{1-\frac{2}{x}+\frac{1}{x^2}} \\ = &-1 && \\ &\lim_{x \to +\infty} \frac{2x-x^2}{x^2-2x+1} \\ = &\lim_{x \to +\infty} \frac{\frac{2}{x}-1}{1-\frac{2}{x}+\frac{1}{x^2}} \\ = &-1 \end{aligned}$$ $$\begin{aligned} g(x) &= 2x-x^2 \\ &= x(2-x) \\ x_0 &\in \{0,2\} \\ && \\ y_0 &= f(0) \\ &= \frac{2(0)-(0)^2}{(0)^2-2(0)+1} \\ &= 0 \end{aligned}$$ $$\begin{aligned} f'(x) &= \frac{(2-2x)(x^2-2x+1) - (2x-x^2)(2x-2)}{(x^2-2x+1)^2} \\ &= \frac{-2 ((x-1)(x^2-2x+1) + (x-1)(2x-x^2))}{(x-1)^4} \\ &= \frac{-2 (x^2-2x+1 + 2x-x^2)}{(x-1)^3} \\ &=-\frac{2}{(x-1)^3} = -2(x-1)^{-3} \\ &\qquad x'_0 \in \{\} \qquad x'^* \in \{1\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) > 0, && x &\in (-\infty,1) && (+) \\ f'(x) < 0, && x &\in (1,+\infty) && (-) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= 6(x-1)^{-4} \\ x''_0 &\in \{\} \qquad x''^* \in \{1\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) > 0, && x &\in (-\infty,1) && (\cup) \\ f''(x) > 0, && x &\in (1,+\infty) && (\cup) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\in \{\} \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\in \{\} \\ \end{aligned}$$
figures/td4-1b.png
$$\frac{2x^2+3x+1}{x^2-1}$$ $$\begin{aligned} f(x) &= \frac{g(x)}{h(x)} \\ h(x) &= x^2-1 \\ &= (x+1)(x-1) \\ x^* &\in \{-1,1\} \\ && \\ \mathbf{X} &= \mathbb{R} \backslash \{-1,1\} \\ \end{aligned}$$ $$\begin{aligned} x^* &\in \{-1,1\} \\ &\lim_{x \to -1} \frac{2x^2+3x+1}{x^2-1} \\ = &\lim_{x \to -1} \frac{(x+1)(2x+1)}{(x+1)(x-1)} \\ = &\lim_{x \to -1} \frac{2x+1}{x-1} = \frac{1}{2}\\ && \\ &\lim_{x \to 1} \frac{2x^2+3x+1}{x^2-1} = \frac{6}{0} \to \infty \\ && \\ x_a &\in \{1\} \\ && \\ y_a &\in \{2\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\infty} \frac{2x^2+3x+1}{x^2-1} \\ = &\lim_{x \to -\infty} \frac{2+\frac{3}{x}+\frac{1}{x^2}}{1-\frac{1}{x^2}} \\ = &2 \\ && \\ &\lim_{x \to +\infty} \frac{2x^2+3x+1}{x^2-1} \\ = &\lim_{x \to -\infty} \frac{2+\frac{3}{x}+\frac{1}{x^2}}{1-\frac{1}{x^2}} \\ = &2 \\ \end{aligned}$$ $$\begin{aligned} g(x) &= 2x^2+3x+1 \\ &= (x+1)(2x+1) \\ x_0 &\in \{-\frac{1}{2}\} \\ && \\ y_0 &= f(0) \\ &= \frac{2(0)^2+3(0)+1}{(0)^2-1} \\ &= -1 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= \frac{(4x+3)(x^2-1)-(2x^2+3x+1)(2x)}{(x^2-1)^2} \\ &= \frac{4x^3-4x+3x^2-3-4x^3-6x^2-2x}{(x+1)^2(x-1)^2} \\ &= \frac{-3x^2-6x-3}{(x+1)^2(x-1)^2} \\ &= \frac{-3(x+1)^2}{(x+1)^2(x-1)^2} \\ &= \frac{-3}{(x-1)^2} = -3(x-1)^{-2} \\ &\qquad x'_0 \in \{\} \qquad x'^* \in \{1\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) < 0, && x &\in (-\infty,1) && (-) \\ f'(x) < 0, && x &\in (1,+\infty) && (-) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= 6(x-1)^{-3} \\ x''_0 &\in \{\} \qquad x''^* \in \{1\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) < 0, && x &\in (-\infty,1) && (\cap) \\ f''(x) > 0, && x &\in (1,+\infty) && (\cup) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\in \{\} \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\in \{\} \\ \end{aligned}$$
figures/td4-1c.png
$$\sqrt{x^2-2x-5}$$ $$\begin{aligned} f(x) &= \sqrt{g(x)}, \quad g(x) \geq 0 \\ g(x) &= x^2-2x-5 = x^2-2x+1-6 \\ &= (x-1+\sqrt{6})(x-1-\sqrt{6}) \\ g''(x) &= 2 > 0 \quad (\cup) \\ &\therefore g(x) > 0, x \in (-\infty, 1-\sqrt{6}) \cup (1+\sqrt{6}, +\infty) \\ && \\ \mathbf{X} &= (-\infty, 1-\sqrt{6}) \cup (1+\sqrt{6}, +\infty) \\ & \approx (-\infty, -1.45) \cup (3.45, +\infty) \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to 1 \pm \sqrt{6}} \sqrt{x^2-2x-5} = 0 \\ \\ x_a &\in \{\} \\ && \\ y_a &\in \{\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\infty} \sqrt{x^2-2x-5} \\ \sim &\lim_{x \to -\infty} \sqrt{x^2} \\ = &+\infty \\ && \\ &\lim_{x \to +\infty} \sqrt{x^2-2x-5} \\ \sim &\lim_{x \to +\infty} \sqrt{x^2} \\ = &+\infty \\ \end{aligned}$$ $$\begin{aligned} \sqrt{g_0} &= 0, \quad g_0 = 0 \\ g(x) &= x^2-2x-5 = 0 \\ &= (x-1+\sqrt{6})(x-1-\sqrt{6}) = 0 \\ x_0 &\in \{1-\sqrt{6}, 1+\sqrt{6}\} \\ && \\ y_0 &= f(0) \\ 0 &\not \in \mathbf{X} \\ &\therefore \text{ no intercept } y_0 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= \frac{1}{2} (x^2-2x-5)^{-\frac{1}{2}} (2x-2) \\ &= (x-1)(x^2-2x-5)^{-\frac{1}{2}} \\ &\qquad x'_0 \in \{\} \qquad x'^* \in \{1-\sqrt{6}, 1+\sqrt{6}\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) < 0, && x &\in (-\infty, 1-\sqrt{6}) && (-) \\ f'(x) > 0, && x &\in (1+\sqrt{6}, +\infty) && (+) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= (1)(x^2-2x-5)^{-\frac{1}{2}} + (x-1)(-\frac{1}{2})(x^2-2x-5)^{-\frac{3}{2}}(2x-2) \\ &= (x^2-2x-5)^{-\frac{1}{2}} -(x-1)^2(x^2-2x-5)^{-\frac{3}{2}} \\ &= \frac{x^2-2x-5}{(x^2-2x-5)^{\frac{3}{2}}} -\frac{x^2-2x+1}{(x^2-2x-5)^{\frac{3}{2}}} \\ &= -\frac{6}{(x^2-2x-5)^{\frac{3}{2}}} \\ &\qquad x'_0 \in \{\} \qquad x'^* \in \{1-\sqrt{6}, 1+\sqrt{6}\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) < 0, && x &\in (-\infty, 1-\sqrt{6}) && (\cap) \\ f''(x) < 0, && x &\in (1+\sqrt{6}, +\infty) && (\cap) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\in \{\} \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\in \{\} \\ \end{aligned}$$
figures/td4-1d.png
$$ln(4x+3)$$ $$\begin{aligned} f(x) &= ln(g(x)), \quad g(x) > 0 \\ g(x) &= 4x+3 > 0 \\ x &> -\frac{3}{4} \\ && \\ \mathbf{X} &= (-\frac{3}{4}, +\infty) \\ \end{aligned}$$ $$\begin{aligned} x_a &\in \{-\frac{3}{4}\} \text{ as found in "Limits at Boundaries"} \\ && \\ y_a &\in \{\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\frac{3}{4}} ln(4x+3) \\ = &-\infty \\ && \\ &\lim_{x \to +\infty} ln(4x+3) \\ = &+\infty \\ \end{aligned}$$ $$\begin{aligned} ln(g_0) &= 0, \quad g_0 = 1 \\ g(x) &= 4x + 3 = 1 \\ x_0 &\in \{-\frac{1}{2}\} \\ && \\ y_0 &= f(0) \\ &= ln(4(0)+3) \\ &= ln(3) \approx 1.098 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= \frac{4}{4x+3} = 4(4x+3)^{-1} \\ &\qquad x'_0 \in \{\} \qquad x'^* \in \{-\frac{3}{4}\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) > 0, && x &\in (-\frac{3}{4}, +\infty) && (+) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= -4(4x+3)^{-2}(4) \\ &= -16(4x+3)^{-2} \\ &\qquad x''_0 \in \{\} \qquad x''^* \in \{-\frac{3}{4}\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) < 0, && x &\in (-\frac{3}{4}, +\infty) && (\cap) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\in \{\} \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\in \{\} \\ \end{aligned}$$
figures/td4-1e.png
$$e^{2x}-3e^x+2$$ $$\begin{aligned} f(x) &= g^2(x)-3g(x)+2 \\ g(x) &= e^x \\ \mathbf{X} &= \mathbb{R} \\ \end{aligned}$$ $$\begin{aligned} x_a &\in \{\} \text{ because } x^* \in \{\} \\ && \\ y_a &\in \{2\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\infty} e^{2x}-3e^x+2 \\ = &(0)-(0)+2 \\ = &2 \\ && \\ &\lim_{x \to +\infty} e^{2x}-3e^x+2 \\ \sim &\lim_{x \to -\infty} e^{2x} \\ = &+\infty \\ \end{aligned}$$ $$\begin{aligned} f(x) &= (e^x)^2-3e^x+2 \\ &= (e^x-2)(e^x-1) \\ x_0 &\in \{0, ln(2)\} \\ &\approx \{0, 0.693\} \\ && \\ y_0 &= f(0) \\ &= e^{2(0)}-3e^{(0)}+2 \\ &= (1)-3(1)+2 \\ &= 0 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= 2e^{2x}-3e^x = 0 \\ 2e^{2x} &= 3e^x \\ e^x &= \frac{3}{2} \\ x'_0 &\in \{ln(\frac{3}{2})\} \\ &\approx \{0.405\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) < 0, && x &\in (-\infty, ln(\frac{3}{2})) && (-) \\ f'(x) > 0, && x &\in (ln(\frac{3}{2}), +\infty) && (+) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= 4e^{2x}-3e^x = 0 \\ 4e^{2x} &= 3e^x \\ e^x &= \frac{3}{4} \\ x''_0 &\in \{ln(\frac{3}{4})\} \\ &\approx \{-0.288\} \\ \end{aligned}$$ $$\begin{aligned} f''(x) < 0, && x &\in (-\infty, ln(\frac{3}{4})) && (\cap) \\ f''(x) > 0, && x &\in (ln(\frac{3}{4}), +\infty) && (\cup) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &= ln(\frac{3}{2}) \\ f''(ln(\frac{3}{2})) &= 4e^{2ln(\frac{3}{2})}-3e^{ln(\frac{3}{2})} \\ &= 4e^{ln(\frac{9}{4})}-3\frac{3}{2} \\ &= 4\frac{9}{4}-\frac{9}{2} \\ &= \frac{9}{2} > 0 \Rightarrow min \\ \end{aligned}$$ $$\begin{aligned} x''_0 &= ln(\frac{3}{4}) \\ \end{aligned}$$
figures/td4-1f.png
$$2sin^2(x)-sin(x)-1$$ $$\begin{aligned} f(x) &= 2g^2(x)-g(x)-1 \\ g(x) &= sin(x) \\ \mathbf{X} &= \mathbb{R} \\ \end{aligned}$$ $$\begin{aligned} x_a &\in \{\} \text{ because } x^* \in \{\} \\ && \\ y_a &\in \{\} \text{ as found in "Limits at Boundaries"} \\ \end{aligned}$$ $$\begin{aligned} &\lim_{x \to -\infty} 2sin^2(x)-sin(x)-1 \\ &\qquad \lim_{x \to -\infty} sin(x) \text{ diverges }\\ && \\ &\lim_{x \to +\infty} 2sin^2(x)-sin(x)-1 \\ &\qquad \lim_{x \to +\infty} sin(x) \text{ diverges }\\ \end{aligned}$$ $$\begin{aligned} f(x) &= 2sin^2(x)-sin(x)-1 \\ &= (2sin(x) + 1)(sin(x) - 1) = 0 \\ sin(x_0) &= -\frac{1}{2} \Rightarrow x_0 \in \{2n\pi+arcsin(-\frac{1}{2}), 2n\pi+\pi-arcsin(-\frac{1}{2})\} \\ &= \{2n\pi-\frac{\pi}{6}, 2n\pi+\pi+\frac{\pi}{6}\} \\ &= \{\pi(2n-\frac{1}{6}), \pi(2n+\frac{7}{6})\}, \qquad n \in \mathbb{Z} \\ sin(x_0) &= 1 \Rightarrow x_0 \in \{2n\pi+arcsin(1), 2n\pi+\pi-arcsin(1)\} \\ &= \{2n\pi+\frac{\pi}{2}, 2n\pi+\pi-\frac{\pi}{2}\} \\ &= \{\pi(2n+\frac{1}{2})\} \\ \therefore x_0 &\in \{(2n-\frac{1}{6})\pi, (2n+\frac{3}{6})\pi, (2n+\frac{7}{6})\pi\} \\ && \\ y_0 &= f(0) \\ &= 2sin^2(0)-sin(0)-1 \\ &= -1 \\ \end{aligned}$$ $$\begin{aligned} f'(x) &= 2(2sin(x))cos(x) - cos(x) \\ &= (4sin(x) - 1) cos(x) = 0 \\ sin(x'_0) &= \frac{1}{4} \Rightarrow x'_0 \in \{2n\pi+\theta, 2n\pi+\pi-\theta\} \\ &\quad \text{ with } \theta=arcsin(\frac{1}{4}) \approx 0.25 \approx \frac{\pi}{12} \\ cos(x'_0) &= 0 \Rightarrow x'_0 \in \{2n\pi+arccos(0), 2n\pi+2\pi-arccos(0)\} \\ &= \{2n\pi+\frac{\pi}{2}, 2n\pi+2\pi-\frac{\pi}{2}\} \\ \therefore x'_0 &\in \{\pi(2n + \frac{\theta}{\pi}), \pi(2n + \frac{1}{2}), \pi(2n + 1 - \frac{\theta}{\pi}), \pi(2n + 2 - \frac{1}{2})\} \\ &\approx \{\pi(2n + \frac{1}{12}), \pi(2n + \frac{6}{12}), \pi(2n + \frac{11}{12}), \pi(2n + \frac{18}{12})\} \\ \end{aligned}$$ $$\begin{aligned} f'(x) > 0, && x &\in (\pi(2n + \frac{1}{12}), \pi(2n + \frac{6}{12})) && (+) \\ f'(x) < 0, && x &\in (\pi(2n + \frac{6}{12}), \pi(2n + \frac{11}{12})) && (-) \\ f'(x) > 0, && x &\in (\pi(2n + \frac{11}{12}), \pi(2n + \frac{18}{12})) && (+) \\ f'(x) < 0, && x &\in (\pi(2n + \frac{18}{12}), \pi(4n + \frac{1}{12})) && (-) \\ \end{aligned}$$ $$\begin{aligned} f''(x) &= (4cos(x))(cos(x)) + (4sin(x) - 1)(-sin(x)) \\ &= 4(cos^2(x) - sin^2(x)) + sin(x) \\ &= 4(1 - sin^2(x) - sin^2(x)) + sin(x) \\ &= -8sin^2(x) + sin(x) + 4 = 0 \Rightarrow \text{ use quadratic formula} \\ sin(x''_0) &= \frac{1-\sqrt{129}}{16} \Rightarrow x''_0 \in \{2n\pi+\alpha, 2n\pi+\pi-\alpha\} \\ &\quad \text{ with } \alpha=arcsin(\frac{1-\sqrt{129}}{16}) \approx -0.70 \approx -\frac{8\pi}{36} \\ sin(x''_0) &= \frac{1+\sqrt{129}}{16} \Rightarrow x''_0 \in \{2n\pi+\beta, 2n\pi+\pi-\beta\} \\ &\quad \text{ with } \beta=arcsin(\frac{1+\sqrt{129}}{16}) \approx 0.88 \approx \frac{10\pi}{36} \\ \therefore x''_0 &\in \{\pi(2n+\frac{\alpha}{\pi}), \pi(2n+\frac{\beta}{\pi}), \pi(2n+1-\frac{\beta}{\pi}), \pi(2n+1-\frac{\alpha}{\pi})\} \\ &\approx \{\pi(2n-\frac{8}{36}), \pi(2n+\frac{10}{36}), \pi(2n+\frac{26}{36}), \pi(2n+\frac{44}{36})\} \end{aligned}$$ $$\begin{aligned} f''(x) > 0, && x &\in (\pi(2n-\frac{8}{36}), \pi(2n+\frac{10}{36})) && (\cup) \\ f''(x) < 0, && x &\in (\pi(2n+\frac{10}{36}), \pi(2n+\frac{26}{36})) && (\cap) \\ f''(x) > 0, && x &\in (\pi(2n+\frac{26}{36}), \pi(2n+\frac{44}{36})) && (\cup) \\ f''(x) < 0, && x &\in (\pi(2n+\frac{44}{36}), \pi(4n-\frac{8}{36})) && (\cap) \\ \end{aligned}$$ $$\begin{aligned} x'_0 &\approx \pi(2n + \frac{1}{12}) \\ f''(\frac{\pi}{12}) &> 0 \Rightarrow min \\ x'_0 &\approx \pi(2n + \frac{6}{12}) \\ f''(\frac{6\pi}{12}) &< 0 \Rightarrow max \\ x'_0 &\approx \pi(2n + \frac{11}{12}) \\ f''(\frac{11\pi}{12}) &> 0 \Rightarrow min \\ x'_0 &\approx \pi(2n + \frac{18}{12}) \\ f''(\frac{18\pi}{12}) &< 0 \Rightarrow max \\ \end{aligned}$$ $$\begin{aligned} x''_0 &\approx \pi(2n-\frac{8}{36}) \\ x''_0 &\approx \pi(2n+\frac{10}{36}) \\ x''_0 &\approx \pi(2n+\frac{26}{36}) \\ x''_0 &\approx \pi(2n+\frac{44}{36}) \\ \end{aligned}$$
figures/td4-1g.png

Problems

Exercise 2

A person's blood glucose level after a meal varies according to the function (...) and $A > 0$ is constant.
$$ g(t) = A(1 + e^{-3t} - e^{-5t}) $$
What is the initial blood glucose level?
$$ g(0) = A(1 + e^0 - e^0) = A $$
What is the limit of the blood glucose as $t \to \infty$?
$$\begin{aligned} \lim_{t \to +\infty} & A(1 + e^{-3t} - e^{-5t}) \\ = & A (1 + e^{-\infty} - e^{-\infty}) \\ = & A (1 + 0 - 0) = A \\ \end{aligned}$$
What is the maximum blood glucose level, and at what time is it reached?
$$\begin{aligned} g'(t) &= A(-3e^{-3t} + 5e^{-5t}) = 0 \\ 5e^{-5t} &= 3e^{-3t} \\ 5 &= 3e^{2t} \\ e^{2t} &= \frac{5}{3} \\ 2t &= ln(\frac{5}{3}) \\ \therefore t^* &= \frac{1}{2} ln(\frac{5}{3}) \\ &= ln(\sqrt{\frac{5}{3}}) \approx 0.26 \\ \end{aligned}$$ $$\begin{aligned} g^* = g(t*) &= A(1 + e^{-3ln(\sqrt{\frac{5}{3}})} -e^{-5ln(\sqrt{\frac{5}{3}})})\\ &= A(1 + \frac{5}{3}^\frac{-3}{2} - \frac{5}{3}^\frac{-5}{2}) \\ &\approx 1.19 A \end{aligned}$$
Plot the curve of $g(t)$ for $t \in [0, \infty)$.
figures/td4-2.png

Exercise 3

Two bacterial colonies grow in the same Petri dish and share the same food supply. The first colony grows according to the law (...) The second colony grows according to the law (...) The initial number of bacteria in each colony is 6 (in units of thousands of cells). Time is measured in days. At time $t = 4$ days, the food supply is removed, and the colonies begin to die.
$$\begin{aligned} b_1(t) &= -t^3 + 5t^2 - 3t + A_0 \\ b_2(t) &= 3t + A_0 \\ b_1(0) &= b_2(0) = 6 \\ t &\in [0,4] \\ \end{aligned}$$
Show, using the initial conditions, that $A_0 = 6$.
$$\begin{aligned} b_1(0) &= 6 \\ -(0)^3 + 5(0)^2 - 3(0) + A_0 &= 6 \\ A_0 &= 6 \\ && \\ b_2(0) &= 6 \\ 3(0) + A_0 &= 6 \\ A_0 &= 6 \\ \end{aligned}$$
Calculate the derivatives of $b_1$ and $b_2$. At time $t = 0$, which population grows the fastest?
$$\begin{aligned} b_1'(t) &= -3t^2 + 10t - 3 \\ b_2'(t) &= 3 \\ b_1'(0) = -3 &< b_2'(0) = 3 \\ \end{aligned}$$ $$\text{In other words, }b_2\text{ grows faster than }b_1\text{ at }t_0$$
During the phase when food is present, determine the maximum and minimum values of each population. At what times are these values reached?
$$\begin{aligned} b_1'(t) &= -3t^2 + 10t - 3 = 0 \\ &= (3t-1)(t-3) = 0 \\ t'_0 &\in \{\frac{1}{3}, 3 \} \\ && \\ b_1''(t) &= -6t + 10 \\ b_1''(\frac{1}{3}) &= 8 > 0 \Rightarrow \text{min} \\ b_1''(3) &= -8 < 0 \Rightarrow \text{max} \\ && \\ b_1(\frac{1}{3}) &= -(\frac{1}{3})^3 + 5(\frac{1}{3})^2 - 3(\frac{1}{3}) + 6 \\ &=-\frac{1}{27}+\frac{5}{9}+5 = \frac{149}{27} \approx 5.52 \\ b_1(3) &= -(3)^3 + 5(3)^2 - 3(3) + 6 \\ &= -27 + 45 - 9 + 6 = 15 \\ && \\ b_2'(t) &= 3 \neq 0 \therefore \text{ no min/max under this criterion } \\ \end{aligned}$$ In other words, $b_1$ reaches a minimum at $t = \frac{1}{3}$ with a population of $\frac{149}{27} \approx 5.52$. It then reaches a maximum at $t = 3$ with a population of $15$. On the other hand, $b_2$ grows linearly, so it doesn't have a point where the derivative changes sign and therefore has neither minima nor maxima using this criterion. However, taking into acount this constant linear growth and that the domain is $t \in [0,4]$, it can be easily seen that the values $b_2(0) = 6$ and $b_2(4) = 18$ correspond to the actual minimum and maximum respectively for this function..