Q:

Drawing multiple circles on Tikz

I want to draw a unit circle with multiple circles with centred on (cos(x),sin(x)).  
\begin{tikzpicture}
\draw (0,0) circle (2cm);

\coordinate (a) at (2;10);

\draw (a) circle (0.3cm);

\end{tikzpicture}

How can I define points using polar coordinates to draw a circle at (2cos(10),2sin(10))? 

A:

Without polar coordinates, this can easily be done as follows
\documentclass[border={10}]{standalone}

\usepackage{tikz}  

\begin{document}

\def \f {1.0}

\begin{tikzpicture}

\foreach \a in {0, 30, ..., 350 }
        \draw ({\f*cos(\a)}, {\f*sin(\a)}) circle (0.2cm);

\end{tikzpicture}

\end{document}

The result is 

A:

With polar coordinates this can also be done :)
 \draw (\a:1) circle (0.2cm);

The syntax is (<angle>:<radius>).  The output is

% arara: pdflatex
% ! arara: indent: {overwrite: yes}
\documentclass[border=3mm]{standalone}
\usepackage{tikz}  
\begin{document}

\begin{tikzpicture}
 \foreach \a in {0, 30, ..., 350 }
 \draw (\a:1) circle (0.2cm);
\end{tikzpicture}

\end{document}

Or, alternatively, using the graphs library, allows you to use
\graph[nodes={draw,circle,minimum width=.2cm},
  clockwise,
  radius=1cm,
  empty nodes,
  n=12]{subgraph I_n};

here's a complete MWE:
% arara: pdflatex
% ! arara: indent: {overwrite: yes}
\documentclass[border=3mm]{standalone}
\usepackage{tikz}  
\usetikzlibrary{graphs}
\usetikzlibrary{graphs.standard}

\begin{document}

\begin{tikzpicture}
 \graph[nodes={draw,circle,minimum width=.2cm},clockwise,radius=1cm,empty nodes,n=12]{subgraph I_n};
\end{tikzpicture}

\end{document}

For reference, see Where can I find a complete list of the standard subgraphs offered by tikz?