Como corrigir esse erro de controle indefinido no meu documento?

Question

\documentclass[landscape,twocolumn,letterpaper]{article}
\usepackage[landscape,twocolumn]{geometry}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage[centertags]{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{epsfig}
\usepackage{graphicx}
\usepackage{amsthm}
\usepackage{amssymb,latexsym}
\usepackage{amsfonts,amsmath}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{amsmath,amssymb}
\usepackage{latexsym}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage[arrow,frame,matrix]{xy}
\usepackage{amsmath}
\usepackage{fixltx2e}
\theoremstyle{definition}
\newtheorem{Thm}{Theorem}[section]
\newtheorem{lem}[Thm]{Lemma}
\newtheorem{pro}[Thm]{Proposition}
\newtheorem{de}[Thm]{Definition}
\newtheorem{re}[Thm]{Remark}
\newtheorem{ex}[Thm]{Example}
\newtheorem{cor}[Thm]{Corollary}
\numberwithin{equation}{section}
\begin{document}
\tableofcontents
\chapter{Complex Numbers}
Complex numbers play important role in mathematics and in sciences. The use of complex numbers is indispensable in Physics.
Real Numbers: All numbers whose square is positive $(x^2?0)$ are known as real     numbers.
Imaginary Numbers: Besides real numbers there are numbers whose square gives negative $(x^2<0)$ are called imaginary numbers.
For example, consider the following equations:
$x^2+1=0$,$x^2+4=0$,$x^2+5=0$,…
The equation  $x^2+1=0$
Or has no solution among the real numbers. Thus we can not find a real number that can satisfy the above equation. Similarly, $x^2+4=0$,$x^2+5=0$ and so on.
To resolve this issue, Mathematicians introduced a new number denoted by a Greek letter of alphabet i(iota) such that $i=?(-1)$ which is called the imaginary unit. If we square both sides of Eq. (1), we have $i^2=-1$ a result that can not be obtained with real numbers. Using this new new number iota we can also define the imaginary numbers as: `` All numbers having iota in their factorization are called imaginary numbers.'' A simple consequences of the definition of iota is that all powers of iota may be expressed in terms of ±1 or i itself.

%\begin{figure}[h]
%\centering
%\includegraphics[scale=1]{Noor1.eps}
%\end{figure}
\end{document} 

Answer 1

A mensagem de erro que você recebe é:

! Undefined control sequence. l.35 \chapter

Você recebe esta mensagem de erro porque não pode usar chapterem combinação com a articledocumentclass.

Você tem as seguintes 2 possibilidades:

1. use a report documentclass em vez disso.
2. fique com a articledocumentclass e substitua \chapterpor \section.