How to use pi(π) symbol in LaTeX?

\documentclass{article}
\begin{document}
   \[ \pi = 3.141\dots \]
   \[ C = 2\pi r \]
   \[ A = \pi r^2 \]
\end{document}

Output :

Related posts :

• Big Pi or pord(Π) symbol in LaTeX
• Sigma, summation(Σ) symbol in LaTeX