Math font family for Merriweather (black) font family

Question

I want to use a strong font in my text, like \usepackage[black]{merriweather}, but it also has a lot of equations. So, is there any "strong" math font I could use?

\documentclass[11pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage[T1]{fontenc}        
\usepackage[black]{merriweather}
\usepackage{lipsum} 
\begin{document}

\chapter{Introduction}
\lipsum[2] $x=4$.

\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

\end{document}

I can not use XeTeX or LuaTex.

Answer 1

If you want to use fouriernc for maths but Merriweather for text, just load fouriernc before merriweather:

\documentclass[11pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage[T1]{fontenc}
\usepackage{fouriernc}
\usepackage[black]{merriweather}
\usepackage{lipsum}
\begin{document}

\chapter{Introduction}
\lipsum[2] $x=4$.

\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

\end{document}

EDIT

It is very difficult to scale the fonts for maths straightforwardly, but, as you pointed out, it is possible to alter the point sizes LaTeX uses for maths using code from Stefan Kottwitz's answer.

For example, to use the settings which are default for 12pt text in an 11pt document:

\documentclass[11pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath,amssymb}
\usepackage{fouriernc}
\usepackage[black]{merriweather}
\usepackage{lipsum}
\makeatletter
\DeclareMathSizes{\@xipt}{\@xiipt}{8}{6}% modified from fontmath.ltx
\makeatother
\begin{document}
\chapter{Introduction}
\lipsum[2] $x=4$.

\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

\[
\int_0^\infty \frac{\sum_{n=-\infty}^{\infty}\frac{1}{n! + \dots 1}}{\prod_{k=i^{j^n}} \exp^k - \pi\delta }
\]

\end{document}

Note that, unlike font scaling, this must be done for every fontsize context in which you will use mathematics.

To illustrate, suppose that we'd set maths to be huge:

\DeclareMathSizes{\@xipt}{\@xxvpt}{\@xxpt}{\@xviipt}

(These sizes correspond to the largest text size LaTeX uses by default.)

Now suppose that we typeset the following

\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

\small
\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

Normally, we'd expect the second to be slightly smaller than the first. For example, with the default settings

However, with our enormous maths we will get the following mismatch

This is because \small alters the current text size and LaTeX chooses the sizes for maths according to the current text size. Because we didn't alter the sizes declared for \small, the default, much, much smaller settings are used.

Similarly, \large will have somewhat counterintuitive effects

\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

\large
\begin{displaymath}
\Delta G^{\circ}= -R\,T\ln \dfrac{[S_1][S_2]}{[S_1S_2]}.
\end{displaymath}

That is, \large makes text bigger and maths smaller.

To avoid this, it would be best to redefine all the standard sizes LaTeX uses proportionately to avoid ending up with subtle inconsistencies in the size of maths.