$\DeclareMathOperator{\fsin}{\text{ημ}}$
$\DeclareMathOperator{\fcos}{\text{συν}}$
$\DeclareMathOperator{\ftan}{\text{εφ}}$
$\DeclareMathOperator{\fcot}{\text{σφ}}$
$\newcommand{\lleft}{\mathopen{}\mathclose\bgroup\left}$
$\newcommand{\mmiddle}{\mathrel{}\middle}$
$\newcommand{\rright}{\aftergroup\egroup\right}$
$\newcommand{\fd}[1]{\mathop{}\!\mathrm{d}{#1}}$