How to substitute the numbers a, b into an expression?
add tag
Laurenso 
I have an expression `x^2 - a x - b x + ab`. Now I want to substitute `a = 1` and `b = 2` to get `x^2 - x - 2x + 2`. How can I get the result?
```
\documentclass[12pt,a4paper]{article}
\usepackage{mathtools}
\usepackage{amsthm}
\usepackage{calc}
\begin{document}
\pgfmathsetmacro{\a}{1}
\pgfmathsetmacro{\b}{2}
x^2 - \a*x - \b*x + \a*\b
\end{document}
```
I can not get the result `x^2 - x - 2x + 2`.
Top Answer
samcarter 
You could wrap your coefficients into a macro to evaluate them:

```
\documentclass{article}

\usepackage{pgf}

\pgfkeys{/pgf/number format/print sign=true}

\newcommand{\foo}[1]{%
  \pgfmathparse{#1}%
  \pgfmathprintnumber{\pgfmathresult} 
}

\begin{document}
\pgfmathsetmacro{\a}{1}
\pgfmathsetmacro{\b}{2}
\[
  x^2 \foo{-\a}x \foo{-\b}x \foo{+\a*\b}
\]
\end{document}
```

![Screenshot 2023-07-23 at 15.12.35.png](/image?hash=aea97c022dc5f2225b8df4a44e75f78f4f321c04aefecfbdc2ce744eea34fc9d)
comments
transcript

Enter question or answer id or url (and optionally further answer ids/urls from the same question) from

Separate each id/url with a space. No need to list your own answers; they will be imported automatically.