\documentclass[12pt]{jsarticle}
\usepackage{amsmath,amssymb}
\pagestyle{empty}
\begin{document}
\setlength{\baselineskip}{22pt}
\begin{flushleft}
{\huge 3}
　角$\alpha$，$\beta$，$\gamma$が$\alpha+\beta+\gamma={180}^\circ$，
$\alpha\geqq0^\circ$，$\beta\geqq0^\circ$，$\gamma\geqq0^\circ$を満たすとき，
$\cos\alpha+\cos\beta+\cos\gamma\geqq1$を示せ．
\end{flushleft}
\end{document}
