diff --git a/Documents/LaTeX/Homework 5.tex b/Documents/LaTeX/Homework 5.tex index 5cae25e..b20fd78 100644 --- a/Documents/LaTeX/Homework 5.tex +++ b/Documents/LaTeX/Homework 5.tex @@ -191,7 +191,7 @@ \end{enumerate} \item $a_1 = 1,\ a_{n+1}=\frac{a_n^2+5}{2a_n}$ - \\\\The first 5 terms of this sequence are $1, 3, \frac{7}{3}, \frac{47}{21}, \frac{2207}{987}, \dots$. This is an increasing sequence. + \\\\The first 5 terms of this sequence are $1, 3, \frac{7}{3}, \frac{47}{21}, \frac{2207}{987}, \dots$. This is a decreasing sequence. \\\\First, we must find the possible limits (fixed points) of the sequence. So, \begin{align*} a&=\frac{a^2+5}{2a} \\ @@ -200,18 +200,68 @@ a &= \pm \sqrt{5} \end{align*} Since we're given that $a_1=1$, we know that the most likely lower bound will be $\sqrt{5}$. - \\\\Now we want to show that $(a_n)$ is bounded below by $-\sqrt{5}$. + \\\\Now we want to show that $(a_n)$ is bounded below by $\sqrt{5}$. \begin{proof} - We want to show that $a_n \leq \sqrt{5},\ \forall\ n \in \N$. We prove it by method of mathematical induction. - \\\\\textbf{Basis Step:} Since $1 \geq -\sqrt{5}$, we have that $a_1 \geq -\sqrt{5}$ - \\\\\textbf{Inductive Step:} Assume that $a_n \geq -\sqrt{5}\ \forall\ n \in \N$. - \\\\\textbf{Show:} We want to show that $a_{n+1} \geq -\sqrt{5}\ \forall\ n \in \N$. So, + We want to show that $a_n \geq \sqrt{5},\ \forall\ n \in \N$. We prove it by method of mathematical induction. + \\\\\textbf{Basis Step:} Since $1 \geq \sqrt{5}$, we have that $a_1 \geq \sqrt{5}$ + \\\\\textbf{Inductive Step:} Assume that $a_n \geq \sqrt{5}\ \forall\ n \in \N$. + \\\\\textbf{Show:} We want to show that $a_{n+1} \geq \sqrt{5}\ \forall\ n \in \N$. So, + \[a_{n+1} = \frac{a_n^2 + 5}{2a_n}\] \begin{align*} - a_{n+1} &= \frac{a_n^2 + 5}{2a_n} + (a_n-\sqrt{5})^2 &\geq 0 \\ + a_n^2 -2\sqrt{5}a_n +5 &\geq 0 \\ + a_n^2 +5 &\geq 2\sqrt{5}a_n \\ + \Downarrow \\ + \frac{a_n^2+5}{2a_n} &\geq \frac{2\sqrt{5}a_n}{2a_n} \\ + \frac{a_n^2+5}{2a_n} &\geq \sqrt{5} \\ + a_{n+1} \geq \sqrt{5} \end{align*} + Thus we have that $(a_n)$ is bounded below by $\sqrt{5}$. \end{proof} + Now we must show that $(a_n)$ is monotone decreasing.\\ + \begin{proof} + We want to show that $(a_n)$ is monotone decreasing; that is, we want to show that $(a_2 \geq a_3 \geq \dots \geq a_n),\ \forall\ n \geq 2$. We prove it by method of mathematical induction. + \\\\\textbf{Basis Step:} Since $3 \geq \frac{7}{3}$, we have that $a_2 \geq a_3$. + \\\\\textbf{Inductive Step:} Assume that $a_n \geq a_{n+1},\ \forall\ n \geq 2$. + \\\\\textbf{Show:} We want to show that $a_{n+2} \leq a_{n+1},\ \forall\ n \geq 2$. + \\So, + \[a_{n+2} = \frac{a_{n+1}^2+5}{2a_{n+1}} \leq \frac{a_n^2+5}{2a_n}\] + Since we have: + \begin{align*} + a_{n+1} &\geq \sqrt{5}, &\text{by the previous proof of boundedness} \\ + a_{n+1}^2 &\geq 5 + \end{align*} + We can equivalently write the inequality as + \[\frac{a_{n+1}^2+5}{2a_{n+1}} \leq \frac{a_{n+1}^2+a_{n+1}^2}{2a_{n+1}}=a_{n+1}\] + Thus we have that $(a_n)$ is monotone decreasing. + \end{proof} + Since $(a_n)$ is both monotone decreasing and bounded, we have + \begin{align*} + \lim (a_n) &= \inf \{a_n:n \in \N\} \\ + &= \sqrt{5} + \end{align*} - \item $a_1 = 5,\ a_{n+1}=\sqrt{4+a_n}$ + \item $a_1 = 5,\ a_{n+1}=\sqrt{4+a_n}$ + \\\\The first 5 terms of this sequence are $5, \sqrt{5}, \sqrt{7}, \frac{\sqrt{57}}{3}, \frac{\sqrt{2751}}{21}, \dots$. This sequence is decreasing. + \\\\First, we must find the possible limits (fixed points) of the sequence. So, + \begin{align*} + a&=\sqrt{4+a} \\ + a^2 &= 4 + a \\ + a^2 -a - 4 &= 0 \\ + \end{align*} + So we have that $a=\frac{1}{2} - \sqrt{17}$, or $a=\frac{1}{2}+\sqrt{17}$. Since we're given that $a_1=5$, we infer that $\frac{1}{2}+\sqrt{17}$ is a possible limit (fixed point) of $(a_n)$. + \\\\Now we want to show that $(a_n)$ is bounded below by $\frac{1}{2}+\sqrt{17}$. + \begin{proof} + We want to show that $a_n \geq \frac{1}{2}+\sqrt{17},\ \forall\ n \in \N$. We prove it by method of mathematical induction. + \\\\\textbf{Basis Step:} $5 \geq \frac{1}{2}+\sqrt{17}$ yields that $a_1 \ge \frac{1}{2}+\sqrt{17}$ + \\\\\textbf{Inductive Step:} Assume $a_n \geq \frac{1}{2}+\sqrt{17},\ \forall\ n \in \N$. + \\\\\textbf{Show:} We want to show that $a_{n+1} \geq \frac{1}{2}+\sqrt{17},\ \forall\ n \in \N$. + \\So, + \begin{align*} + a_{n+1} \ge \sqrt{4+a_n} + \end{align*} + + \end{proof} \end{enumerate} \item