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\documentclass[12pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot,showframe]{geometry}
\usepackage{setspace}
\setstretch{1.1}
\raggedbottom
\usepackage{microtype}
\usepackage[bottom]{footmisc} % Options : [ perpage ], [ symbol ], [ multiple ]
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{mathtools}
\begin{document}
Blablabla\footnote{Blablabla :
\begin{align*}
x &= \frac{A}{B},
& y &= \frac{C}{D}.
\end{align*}
}
\end{document}
Adding the command \vspace{-12pt} just under the align environment is a solution, but it feels like an ugly hack to me. Is there a better solution?