diff --git a/ma1522/assignment/assignment2.pdf b/ma1522/assignment/assignment2.pdf new file mode 100644 index 0000000..eeb2aea Binary files /dev/null and b/ma1522/assignment/assignment2.pdf differ diff --git a/ma1522/assignment/assignment2.tex b/ma1522/assignment/assignment2.tex new file mode 100644 index 0000000..4297dca --- /dev/null +++ b/ma1522/assignment/assignment2.tex @@ -0,0 +1,176 @@ +\documentclass[a4paper]{article} + +\input{../preamble.tex} + + + +% ------------------------------------------------------------------------------ + +\begin{document} + +% ------------------------------------------------------------------------------ +% Cover Page and ToC +% ------------------------------------------------------------------------------ + +\title{\normalsize \textsc{} + \\ [2.0cm] + \HRule{1.5pt} \\ + \LARGE \textbf{\uppercase{MA1522} + \HRule{2.0pt} \\ [0.6cm] \LARGE{Assignment 2} \vspace*{10\baselineskip}} + } +\date{} +\author{\textbf{Yadunand Prem}\\ +A0253252M} + +\maketitle +\newpage + +% ------------------------------------------------------------------------------ + +\section{Question 1} +\hr +\begin{align*} + \overrightarrow{A} = \begin{pmatrix} + 2 & -4 & 4 & -2 \\ + 6 & a-12 & 7 & a-6 \\ + -1 & 2-b & 8 & -b+1 \\ + \end{pmatrix} + \xrightarrow[R_3 + \frac{1}{2}R_1]{R_2 + (-3)R_1} + & \begin{pmatrix} + 2 & -4 & 4 & -2 \\ + 0 & a & -5 & a \\ + 0 & -b & 10 & -b \\ + \end{pmatrix} \\ + \xrightarrow{R_3 + \frac{b}{a}R_2} + & \begin{pmatrix} + 2 & -4 & 4 & -2 \\ + 0 & a & -5 & a \\ + 0 & 0 & 10-\frac{5b}{a} & 0 \\ + \end{pmatrix} = \overrightarrow{U} +\end{align*} + +\begin{align*} + \overrightarrow{I} \xrightarrow{R_3 + \frac{-b}{a}R_2} \xrightarrow[R_3 + \frac{-1}{2}R_1]{R_2 + (3)R_1} \overrightarrow{L} +\end{align*} + +\begin{align*} + \begin{pmatrix} + 1 & 0 & 0 \\ + 0 & 1 & 0 \\ + 0 & 0 & 1 + \end{pmatrix} + \xrightarrow{R_3 + (-\frac{b}{a})R_2} + \begin{pmatrix} + 1 & 0 & 0 \\ + 0 & 1 & 0 \\ + 0 & -\frac{b}{a} & 1 + \end{pmatrix} + \xrightarrow[R_3 + \frac{-1}{2}R_1]{R_2 + (3)R_1} + \begin{pmatrix} + 1 & 0 & 0 \\ + 3 & 1 & 0 \\ + -\frac{1}{2} & -\frac{b}{a} & 1 + \end{pmatrix} = \overrightarrow{L} +\end{align*} +If $a = 0$, then in the 1st step of the factorization above, it will cause the array to pivot. Thus, to prevent pivoting, $b = 0$ must be the case also. +$a \neq 0$ then $b \neq 0$ also, as $\frac{b}{a}$ will not be defined + +\newpage +\section{Question 2} +\hr +For A to be invertible, $det(A) \neq 0$. + +$A = \begin{vmatrix} + 1 & 1 & 1 & 1 \\ + 1 & 2 &-1 & c \\ + 1 & 4 & 1 & c^2 \\ + 1 & 8 &-1 & c^3 \\ +\end{vmatrix} +$ + +\begin{align*} + det(A) + & = (1)\begin{vmatrix} + 2 &-1 & c \\ + 4 & 1 & c^2 \\ + 8 &-1 & c^3 \\ + \end{vmatrix} + + (-1)\begin{vmatrix} + 1 &-1 & c \\ + 1 & 1 & c^2 \\ + 1 &-1 & c^3 \\ +\end{vmatrix} + +(1)\begin{vmatrix} + 1 & 2 & c \\ + 1 & 4 & c^2 \\ + 1 & 8 & c^3 \\ +\end{vmatrix} + +(-1)\begin{vmatrix} + 1 & 2 &-1 \\ + 1 & 4 & 1 \\ + 1 & 8 &-1 \\ +\end{vmatrix} \\ + & = (1)\left((2)\begin{vmatrix} + 1 & c^2 \\ + -1 & c^3 \\ + \end{vmatrix} + + (1)\begin{vmatrix} + 4 & c^2 \\ + 8 & c^3 \\ + \end{vmatrix} + + (c)\begin{vmatrix} + 4 & 1 \\ + 8 &-1 \\ + \end{vmatrix}\right)\\ + & + (-1)\left((1)\begin{vmatrix} + 1 & c^2 \\ + -1 & c^3 \\ + \end{vmatrix} + + (1)\begin{vmatrix} + 1 & c^2 \\ + 1 & c^3 \\ + \end{vmatrix} + + (c)\begin{vmatrix} + 1 & 1 \\ + 1 &-1 \\ + \end{vmatrix}\right)\\ + & + (1)\left((1)\begin{vmatrix} + 4 & c^2 \\ + 8 & c^3 \\ + \end{vmatrix} + + (-2)\begin{vmatrix} + 1 & c^2 \\ + 1 & c^3 \\ + \end{vmatrix} + + (c)\begin{vmatrix} + 1 & 4 \\ + 1 & 8 \\ + \end{vmatrix}\right)\\ + & + (-1)\left((1)\begin{vmatrix} + 4 & 1 \\ + 8 &-1 \\ + \end{vmatrix} + + (-2)\begin{vmatrix} + 1 & 1 \\ + 1 &-1 \\ + \end{vmatrix} + + (-1)\begin{vmatrix} + 1 & 4 \\ + 1 & 8 \\ + \end{vmatrix}\right)\\ + & = (2)(c^3+c^2) + (4c^3-8c^2) + (c)(-4-8)\\ + & + (-1)((c^3+c^2) + (c^3-c^2) + (c)(-1 - 1))\\ + & + (4c^3-8c^2) + (-2)(c^3-c^2) + (c)(8-4)\\ + & + (-1)((-4-8) + (-2)(-1-1) + (-1)(8-4)\\ + & = 6c^3 - 12c^2 - 6c + 12 \\ + & = 6(c - 2)(c - 1)(c + 1) +\end{align*} + +Therefore, $det(A) = (c-2)(c-1)(c+1) \neq 0$, \\ +$c \neq 2, c \neq 1, c \neq -1$ + +Actually, this question can be solved by observation. The determinant will be 0 if 2 columns are a multiple of one another. By observation, we can see that if $c = 1$, then the 4th and 1st column will be the same. If $c = 2$, then 2,4 and $c=-1$, then 3,4 will be the same. + +% ------------------------------------------------------------------------------ + +\end{document}