[Tex/LaTex] Texmaker \color or \textcolor: no line to end error

colorerrors

I'm new to LaTeX and I don't get where my error is. I'm running on Windows under Texmaker.
I only recieve this message : "! LaTeX Error: There's no line here to end."

\documentclass[12pt,a4paper]{article}
\usepackage[top=1cm,bottom=1cm,left=1cm,right=1cm]{geometry}
\usepackage{calc,multido,ifthen,bm}
\usepackage{graphics} 
\usepackage[french]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{fancyhdr}
\usepackage{stmaryrd}
\usepackage{setspace}
\usepackage{dsfont}
\usepackage{enumitem}
\usepackage{natbib}
\usepackage{mathtools, amssymb,amsthm}
\usepackage{xcolor}
\title{Sujet 4}
\begin{document}
\section {Exercice 4 :} Soit $P(x) = x^3 - x + 1 $.
\\
txt
\\
\textcolor{blue} {On sait que $\alpha$, $\beta$, $\overline{\beta}$ sont les 3 racines du polynômes P. Donc P peut s'ecrire sous forme scindée : 
$$P(x)=(x-\alpha)(x-\beta)(x-\overline{\beta})$$
$P(x)=(x^2-\beta x-\alpha x+\alpha \beta)(x-\overline{\beta})$ en développant
$$P(x)=x^3-\beta x^2 - \alpha x^2 +\alpha \beta x -\overline{\beta} x^2 +\beta \overline{\beta} x + \alpha \overline{\beta} x - \alpha \beta \overline{\beta}$$ 
Par identification, on obtient notamment :
\[-\alpha \beta \overline{\beta} = 1$$
$$\lvert \alpha \beta \overline{\beta} \rvert = 1$$
$$\lvert \alpha \rvert \lvert \beta \rvert \lvert \overline{\beta} \rvert = 1$$
$$\lvert \alpha \rvert \lvert \beta \rvert^2 = 1$$
$$\lvert \beta \rvert=\frac{1}{\lvert \alpha \rvert}\]
or $\alpha <-1$ donc $\lvert \alpha \rvert>1$
donc $\lvert \beta \rvert<1$}
\end{document}

There, I've done what you have said. I tried to run it but I had the same error in the same line.

Thank you for your help !

Best Answer

let me elaborate my comment (as supplement to samcarter answer): \textcolor{<color>}{.....} doesn't work over more paragraphs (as is your case). instead you should rather use switch \color{<color>} and after end of colored part of document switch back with \color{black}:

enter image description here

\documentclass[12pt,a4paper]{article}
\usepackage[top=1cm,bottom=1cm,left=1cm,right=1cm]{geometry}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{natbib}
\usepackage[french]{babel}
\usepackage{calc,multido,ifthen,bm}
\usepackage{graphicx}
\usepackage{fancyhdr}
\usepackage{stmaryrd}
\usepackage{setspace}
\usepackage{dsfont}
\usepackage{enumitem}
\usepackage{mathtools, amssymb, amsthm}
\usepackage{xcolor}
\title{Sujet 4}

\begin{document}
\section {Exercice 4 :} Soit $P(x) = x^3 - x + 1 $.
\\
txt
\\
\color{blue}
On sait que $\alpha$, $\beta$, $\overline{\beta}$ sont les 3 racines du polynômes P. Donc P peut s'ecrire sous forme scindée :
    \[
P(x)=(x-\alpha)(x-\beta)(x-\overline{\beta})
    \]
$P(x)=(x^2-\beta x-\alpha x+\alpha \beta)(x-\overline{\beta})$ en développant
    \[
P(x)=x^3-\beta x^2 - \alpha x^2 +\alpha \beta x -\overline{\beta} x^2 +\beta \overline{\beta} x + \alpha \overline{\beta} x - \alpha \beta \overline{\beta}
    \]
Par identification, on obtient notamment :
    \begin{align*}
-\alpha \beta \overline{\beta}
    & = 1  \\
\lvert \alpha \beta \overline{\beta} \rvert
    & = 1\\
\lvert \alpha \rvert \lvert \beta \rvert \lvert \overline{\beta} \rvert
    & = 1\\
\lvert \alpha \rvert \lvert \beta \rvert^2
    & = 1\\
\lvert \beta \rvert
    & =\frac{1}{\lvert \alpha \rvert}
    \end{align*}
or $\alpha <-1$ donc $\lvert \alpha \rvert>1$
donc $\lvert \beta \rvert<1$

\color{black}
some text
\end{document}

by the way, Textmaker have nothing with your problem. it is only editor for latex.

Related Question