# Missing { inserted?

I keep getting the error:

Missing { inserted.
<to be read again>


and I don't know what I'm doing wrong. I just started using Latex and have no clue what I'm doing. below is what I have done.

\documentclass{article}

\usepackage[utf8]{inputenc}

\usepackage{amsmath}

\begin{document}

\section{Problem 5}

\begin{align*}

\\ b + 2y &= a

\\ \frac{b}{2} + y &= \frac{a}{2}

\\ y &= \frac{a-b}{2}

\\ 2y &= a-b

\\(x+b)^2 + (\frac{b}{2})^2&=r

\\(a-x)^2 + (\frac{a}{2})^2&=r

\\(x+b)^2 + (\frac{a}{2})^2&=(a-x)^2 + (\frac{a}{2})^2

\\x^2 + 2bx + b^2 + \frac{b^2}{4} &=a^2 -2ax + x^2 + \frac{a^2}{4}

\\2bx + \frac{5b^2}{4} &= -2ax + \frac{5a^}{4}

\\ \frac{2bx+2ax}{2} &= \frac{\frac{5a^2}{4}}-\frac{{5b^2}{4}}{2}

\\ bx + ax &= \frac{5a^2}{8} - \frac{5b^2}{8}

\end{align*}

\end{group*}

\end{document}

• (there is one typo on line 13)
– Jack
May 20 '17 at 15:21
• \\  is end of line not start, so omit the first \\ also you can not have blank lines within the environment. May 20 '17 at 15:23
• please always post full (small) document that shows the problem, your example can not be run to reproduce the error, also what is this enviornment? \end{group*} May 20 '17 at 15:24
• In addition to what David has already pointed out. \\2bx + \frac{5b^2}{4} &= -2ax + \frac{5a^}{4} Did you perhaps forget to type in the superscript for 5a?
– Troy
May 20 '17 at 15:26
• Hi! My teacher put in the \end{group} (but I took it out) and I'm pretty sure the \\ can go at the beginning of a line or the end. I have it at the beginning and all is going well. Thanks!
– Jack
May 21 '17 at 1:22

## 1 Answer

TeX gives you a lot of help to find the problem.

With the code as posted the error is

Runaway argument?

! Paragraph ended before \align* was complete.
<to be read again>
\par
l.12

?


which is telling you that the blank line (paragraph end) on line 12 isn't allowed inside align*.

So if you remove the blank lines and run it again

You get the error

! Missing { inserted.
<to be read again>
\endgroup
l.23 \end{align*}


This is slightly more obscure as for technical reasons in align such errors are always reported as being on the last line, however it tells you that the {} matching is wrong, or actually in this case a^} should have been a^{2}}

then, running it again, the error is

! LaTeX Error: \begin{document} ended by \end{group*}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
...

l.25 \end{group*}


I have no idea what that is intended to be so simply delete that line. It then runs without error but with excessive space before the display as there is an empty row as the first line before the first \\ so re-arranging that produces

\documentclass{article}

\usepackage[utf8]{inputenc}

\usepackage{amsmath}

\begin{document}

\section{Problem 5}

\begin{align*}
b + 2y &= a\\
\frac{b}{2} + y &= \frac{a}{2}\\
y &= \frac{a-b}{2}\\
2y &= a-b\\
(x+b)^2 + (\frac{b}{2})^2&=r\\
(a-x)^2 + (\frac{a}{2})^2&=r\\
(x+b)^2 + (\frac{a}{2})^2&=(a-x)^2 + (\frac{a}{2})^2\\
x^2 + 2bx + b^2 + \frac{b^2}{4} &=a^2 -2ax + x^2 + \frac{a^2}{4}\\
2bx + \frac{5b^2}{4} &= -2ax + \frac{5a^{2}}{4}\\
\frac{2bx+2ax}{2} &= \frac{\frac{5a^2}{4}}-\frac{{5b^2}{4}}{2}\\
bx + ax &= \frac{5a^2}{8} - \frac{5b^2}{8}
\end{align*}

\end{document}