# Error while writing equation

\documentclass[12pt,a4paper]{article}
\usepackage[a4paper, total={6in, 8in}]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{marginnote}
\begin{document}
\underline {Chandrashekhar EOS (1935)} : \\
\begin{align*}
\overline \rho &= $K$ ~\Big( $sinh ~t$ ~- ~$t$\Big)\\
\overline P &= \frac {1}{3} $K$ ~ \Big($sinh~t$ - 8 $sinh$ \frac {1}{2} t  + $3t$\Big)\\
\end {align*}
\end {document}


can you tell me why I have error in the right side of the equations?

• Don't use \$ inside align, as the material is already in math mode. Use \sinh for the hyperbolic sine and \Bigl in front of the opening parentheses, \Bigr with the closing ones (but in those formulas you don't need larger parentheses). Don't use ~ in math mode. – egreg Aug 7 '14 at 10:17

In addition to @egreg's comments, I would suggest to replace \overline here (too large) with widebar, borrowed from the mathx font (mathabx package) and for the fraction coefficients, medium-sized fractiions, defined in the nccmath package. Also, don't load amsmath, since mathtoolsdoes it for you. You can replace some of the ~ with a thin space (\,).

\documentclass[12pt,a4paper]{article}
\usepackage[a4paper, total={6in, 8in}]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{nccmath}

\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{ <->  mathx10 }{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\DeclareMathAccent{\widebar}{0}{mathx}{"73}
\usepackage{marginnote}

\begin{document}

\underline {Chandrashekhar EOS (1935)} :
\begin{align*}
\widebar{\rho} &= K \,\bigl( \sinh t  - t \bigr)\\
\widebar{P} &= \mfrac {1}{3}K\, \Bigl(\sinh t  - 8 \sinh  \mfrac {1}{2} t  +  3t \Bigr)
\end {align*}

\end {document}


• \bigl and \bigr, if you really want larger delimiters. Since you're defining the font shape, use <-> instead of fixed sizes, as mathx10 is available in Type1 format. – egreg Aug 7 '14 at 10:46
• You're right. I'll change it at once. – Bernard Aug 7 '14 at 10:47