# Latex missread the equation and provides an error

I have an equation to be written. The equation I expect to have is this one:

\begin{align}
M &= \left\{
\begin{array}
\sqrt{1+0.6275 \frac{l^2}{Dt} -0.003375 \frac{l^4}{D^2 t^2}} if \frac{l^2}{Dt} \leq 50 \\
0.032 \frac{l^2}{Dt} +3.293 if \frac{l^2}{Dt} > 50
\end{array} \right.

\end{align}

Instead what I get is this:

I don't understand why. Also, I receive an error message when I run the code:

missing delimiter (. inserted)

even if I inserted on purpose

\right. to close it.

What is the problem?

• Shouldn't the \sqrt be outside of the array? I would also use \text{if} from the amsmath package instead of if. – Christian May 16 '16 at 16:23
• Not a case of misreading but of misspelling. – samcarter is at topanswers.xyz May 16 '16 at 16:25

You have a big error: you're missing the mandatory argument to array.

However, there are better methods to accomplish what you want.

\documentclass{article}
\usepackage{amsmath,mathtools}

\begin{document}

\begin{equation*}
M =
\begin{dcases}
\sqrt{1+0.6275 \frac{l^2}{Dt} -0.003375 \frac{l^4}{D^2 t^2}}
& \text{if } \frac{l^2}{Dt} \leq 50 \\
0.032 \frac{l^2}{Dt} +3.293 & \text{if } \frac{l^2}{Dt} > 50
\end{dcases}
\end{equation*}

\end{document}

• For the record, there's also dcases* (and all the other variants) which puts the second column in text mode, writing .. & if $\frac{\ell}{Dt} \leq 50$ \\ feels more natural to me. – Manuel May 16 '16 at 20:35