Find the flaw in this math code

Question

I simply can't find the flaw in this code:

\begin{align}
\begin{split}
x_0'^2-x_1'^2 & = {(x_0 - \beta x_1)}^2 \gamma^2 - {(x_1 - \beta x_0)}^2 
\gamma^2  \\
& = \frac{ (x_0^2 - 2 \beta x_1 x_0 + \beta^2 x_1^2) - (x_^2 - 2\beta x_1 
x_0 + \beta^2 x_0^2)}{1-\beta^2 }
\end{split}
\end{align}

It keeps saying ! Missing { inserted. ^ l.227 \end{align} but I can't see where I went wrong. It has nothing to do with my preamble or other things, I have tried to put something else in there. Everything works fine except for when I write that down. Heeeelp.

Answer 1

In the place of ? you probably wanted to have an index:

\documentclass{article}

\usepackage{amsmath}
\begin{document}



\begin{align}
\begin{split}
{x_0'}^2-{x_1'}^2 & = {(x_0 - \beta x_1)}^2 \gamma^2 - {(x_1 - \beta x_0)}^2 
\gamma^2  \\
& = \frac{ (x_0^2 - 2 \beta x_1 x_0 + \beta^2 x_1^2) - ({x_{?}}^2 - 2\beta x_1 
x_0 + \beta^2 x_0^2)}{1-\beta^2 }
\end{split}
\end{align}

\end{document}

Please observe also that I have added curly braces around expressions with primes, to explain probable intentions.