How to write different cases in equations

Many times we need to write different cases like      

a= {0.6 if t <= 100
      {0.9 it t > 100

The easiest, and most elegant way to do this is to use \case command, as
 $a = \begin{cases}
     0.6, & \textnormal{if } t\le100 \\
     0.9 & \textnormal{if } t>100
\end{cases}$