Align multiline equation in bold font

Question

I want to align an equation:

\begin{align*}
a &= b\\
c &= d
\end{align*}

Which works. But having it in \mathbf{}:

\begin{align*}
\mathbf{a &= b}\\
c &= d
\end{align*}

gives me an error.

Of course, I could end the \mathbf at &= and apply it afterwards, but is there an easier way? (I have long and complicated equations)

Answer 1

you can use \boldmath, but be careful -- this will make all math within its scope bold:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
text $a + b$.
\boldmath
text $a + b$.
\begin{align*}
a &= b\\
c &= d
\end{align*}
\unboldmath
text $a + b$.
\end{document}

you can also limit the scope by placing the intended material within braces.

Answer 2

The error in your example is because the alignment markers & only work inside the same group. So, the easiest solution may be to locally bold the symbols you want using the bm package (better than \mathbf{...}).

\documentclass{article}
\usepackage{amsmath,bm}
\begin{document}

\begin{align*}
  \bm a &=\bm b\\
  c     &=    d
\end{align*}

\end{document}