Q:

Undefined control sequence error \end{split}

I tried to split an equation, such that it alligns under the = sign. I get an error message using this code. Could someone tell me what's wrong?

Undefined control sequence. <argument> ...it@tag \begin {split} TCU &
  = \fraq 
                                                    {\text {purchasing cost} +... l.53 \end{split}

\begin{equation}
\begin{split}
TCU & = \fraq{\text{purchasing cost} +\text{setup cost} + \text{holding cost}}{time} \\
    & = \fraq{cy+K+h\fraq{y}{2}t_0}{t_0} 
\end{split}
\end{equation}

A:

As I said in comment, for fraction is correct command \frac:
    \documentclass{article}
\usepackage{mathtools,amssymb}

    \begin{document}
\begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\frac{y}{2}t_0}{t_0}
\end{split}
\end{equation}    
    \end{document}

The fraction in numerator in the second equation line looks to small (to my taste, so for it I suggest to use \dfrac:
    \documentclass{article}
\usepackage{mathtools,amssymb}

    \begin{document}
\begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\dfrac{y}{2}t_0}{t_0}
\end{split}
\end{equation}    
    \end{document}

which gives:

