begin{subequations}
 begin{equation}
   mid H(jomega) mid = frac{omega CR}{sqrt{1+(omega CR)^2}}
   label{subeqn:amplitude}
 end{equation}
 begin{equation}
   angle H = 90^o - arctan(omega CR)
   label{subeqn:phase}
 end{equation}
end{subequations}

documentclass[12pt,a4paper]{article}
usepackage{amsmath}
begin{document}
begin{subequations}
 begin{gather}
   lvert H(jomega) rvert = frac{omega CR}{sqrt{1+(omega CR)^2}}
   label{subeqn:amplitude}
      angle H = 90^circ - arctan(omega CR)
 label{subeqn:phase}
end{gather}
end{subequations}

end{document}

documentclass[12pt,a4paper]{article}
usepackage[english,greek]{babel}
usepackage[iso-8859-7]{inputenc}
usepackage{kerkis}
usepackage{amsmath}
usepackage{amssymb}
usepackage{amsfonts}
usepackage{amsthm}
usepackage{units}
usepackage{array}
usepackage{framed} % ãéá ãêñé background ðßóù áðü êÜðïéá paragraphs
usepackage{extarrows}
usepackage{cancel}
usepackage[pdftex]{graphicx}
 %usepackage[twoside,pdftex,total={16.4cm,24cm}, left=2.9cm, top=3.2cm, headsep=1.4cm]            {geometry}
%usepackage[top=3.2cm, bottom=3.4cm, left=2.9cm, right=2.9cm,headsep=0.8cm]{geometry}
usepackage[margin=2.8cm]{geometry}
usepackage[Bjornstrup]{fncychap}
 usepackage{fancybox}
usepackage{fancyhdr}
usepackage{colortbl}
usepackage{xcolor}
usepackage{footmisc}
usepackage{subfiles}
usepackage{float}
usepackage{slashbox}
%usepackage{float}
%restylefloat{figure}
%_________________________________________________________________________________________

usepackage{hyperref}
usepackage{subfigure}
renewcommand{labelitemi}{$circ$}
usepackage{wrapfig}
usepackage{color}
%usepackage[usenames,dvipsnames]{color}
definecolor{Darkgreen}{rgb}{0,0.4,0}
definecolor{Darkblue}{rgb}{0,0,0.4}
newcommand{HRule}{rule{linewidth}{0.8mm}}
usepackage{rotating}
%usepackage[landscape]{geometry}
usepackage{lscape}
usepackage{cancel}
usepackage{fancyvrb}
%usepackage{yannisgr}
usepackage{listings}
%lstset{ %
%language=matlab,                % choose the language of the code
%basicstyle=footnotesize,       % the size of the fonts that are used for the code
%numbers=left,                   % where to put the line-numbers
%numberstyle=footnotesize,      % the size of the fonts that are used for the line-     numbers
%stepnumber=1,                   % the step between two line-numbers. If it is 1 each    line will be numbered
%numbersep=5pt,                  % how far the line-numbers are from the code
%backgroundcolor=color{white},  % choose the background color. You must add     usepackage{color}
%showspaces=false,               % show spaces adding particular underscores
%showstringspaces=false,         % underline spaces within strings
%showtabs=false,                 % show tabs within strings adding particular    underscores
%frame=single,           % adds a frame around the code
%tabsize=2,          % sets default tabsize to 2 spaces
%captionpos=b,           % sets the caption-position to bottom
%breaklines=true,        % sets automatic line breaking
%breakatwhitespace=false,    % sets if automatic breaks should only happen at   whitespace
%escapeinside={%*}{*)}          % if you want to add a comment within your code
%}
usepackage{textcomp}
definecolor{listinggray}{gray}{0.9}
definecolor{lbcolor}{rgb}{0.9,0.9,0.9}
lstset{
backgroundcolor=color{lbcolor},
tabsize=4,
rulecolor=,
language=matlab,
    basicstyle=scriptsize,
    upquote=true,
    aboveskip={1.5baselineskip},
    columns=fixed,
    showstringspaces=false,
    extendedchars=true,
    breaklines=true,
    prebreak = raisebox{0ex}[0ex][0ex]{ensuremath{hookleftarrow}},
    frame=single,
    showtabs=false,
    showspaces=false,
    showstringspaces=false,
    identifierstyle=ttfamily,
    keywordstyle=color[rgb]{0,0,1},
    commentstyle=color[rgb]{0.133,0.545,0.133},
    stringstyle=color[rgb]{0.627,0.126,0.941},
}
%usepackage{subeqn}

%_________________________________________________________________________________________


definecolor{mygray}{gray}{.75}%{.9, .9, .9}
definecolor{lightgray}{gray}{.8}%{.9, .9, .9}
definecolor{shadecolor}{gray}{.90} %gia framed package
%definecolor{mygray}{rgb}{.9, .9, .9} %enallaktika !! anti gia panw
newcommand*mygraybox[1]{colorbox{mygray}{hspace{1em}#1hspace{1em}}}

%renewcommand{topfraction}{0.85}%gia na min uparxoun 2 figures se mia selida kai   tipota allo
renewcommand{textfraction}{0.1}
renewcommand{floatpagefraction}{0.75}

newcommand{kai}{quad text{êáé} quad}
newcommand{kkai}{qquad text{êáé} qquad}

linespread{1.1}
newcommand{sw}{selectlanguage{english}}
newcommand{sq}{selectlanguage{greek}}
newcommand{eng}[1]{latintext#1greektext}
newcommand{gre}[1]{greektextlatintext#1}
 newcolumntype{C}[1]{>{centeringletnewline\arraybackslashhspace{0pt}}m{#1}}

begin{comment}
letorigdoublepagecleardoublepage
newcommand{clearemptydoublepage}{%
  clearpage
  {pagestyle{empty}origdoublepage}%
}
letcleardoublepageclearemptydoublepage


pagestyle{fancy}
renewcommand{chaptermark}[1]{markboth{#1}{}}
renewcommand{sectionmark}[1]{markright{thesection #1}}
fancyhf{} % delete current setting for header and footer
fancyhead[LE,RO]{bfseriesthepage}
fancyhead[LO]{rightmark}
fancyhead[RE]{leftmark}
renewcommand{headrulewidth}{0.5pt}
renewcommand{footrulewidth}{0pt}
addtolength{headheight}{0.5pt} % make space for the rule      %Ôï ÷ù ïñßóåé 0.8 áð ôï  package
fancypagestyle{plain}{%
fancyhead{} % get rid of headers on plain pages
renewcommand{headrulewidth}{0pt} % and the line}

newtheorem{thm}{Èåþñçìá}[chapter]
newtheorem{cor}[thm]{Ðüñéóìá}
newtheorem{lem}[thm]{ËÞììá}
newtheorem{parad}{ÐáñÜäåéãìá}[chapter]
newtheorem{defgr}{Ïñéóìüò}[chapter]
end{comment}

parskip 3mm
footskip = 15mm

begin{subequations}
 begin{gather}
   lvert H(jomega) rvert = frac{omega CR}{sqrt{1+(omega CR)^2}}
   label{subeqn:amplitude}
 \
   angle H = 90^circ - arctan(omega CR)
   label{subeqn:phase}
 end{gather}
end{subequations}