how to put left brace in equation? [duplicate]

Question

i want to write like this

        /ki    if 0<I<IO
       | 
t(i)= <
       |
        \c     if I>Io

here is curly bracket

Answer 1

As has already been suggested in the comments, it looks like a cases environment is what you need.

\documentclass{article}
\usepackage{amsmath} % for 'cases' environment
\begin{document}
\[
t(i) = 
\begin{cases}
k_i & \text{if $0<i<i_0$}\\
c   & \text{if $i>i_0$} 
\end{cases}
\]
\end{document}