How to align text to the right in math mode?

Question

I have a table of formulas, some have a description next to them, I want the equations to appear left flush and the text between parentheses right flush. I was writing it in the following way.

\documentclass[letterpaper,10 pt,twocolumn,fleqn]{article}
\setlength{\columnsep}{2.5cm}
\usepackage[inner=2.5cm,outer=2.5cm,bottom=2.5cm,top=2.5cm,landscape]{geometry}
\usepackage{nopageno,array,graphicx,amsbsy,amsfonts} 
\usepackage{amsmath} 
\begin{document}       
\[{\bf P} = \int_{t_i}^{t_f}{\bf F}\,dt = \Delta{\bf p}\mbox{\hspace{7mm} (Impulse) }\]
\[\frac{dA}{dt} = \frac{l}{2\mu}=cte \mbox{\hspace{7mm} (Kepler's Second Law) }\]
\end{document}

but the alignment is different for each equation.

Answer 1

You can use \tag* from the amsmath package.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\mathbf{P} = \int_{t_i}^{t_f}\mathbf{F}\,dt = \Delta \mathbf{p}\tag*{(Impulse) }
\]
\[
\frac{dA}{dt} = \frac{l}{2\mu}=cte \tag*{(Kepler's Second Law) }
\]
\end{document}

Note that \mathbf is preferable to \bf; see this question for more details.

Answer 2

Here's another approach, currently set up to use the full \textwidth.

\documentclass{article}
\newcommand\lreqn[2]{\noindent\makebox[\textwidth]{$\displaystyle#1$\hfill(#2)}\vspace{2ex}}
\begin{document}
\lreqn{\textbf{P} = \int_{t_i}^{t_f}\textbf{F}\,dt = \Delta\textbf{p}}{Impulse}
\lreqn{\frac{dA}{dt} = \frac{l}{2\mu}=cte }{Kepler's Second Law}

This is the paragraph that follow the list
\end{document}

Answer 3

You could put it in a tabular environment

\documentclass{article}
\usepackage{array}
\def\extrarowheight{1.5em}
\let\ds\displaystyle
\begin{document}
\begin{tabular}{lr}
\( \textbf{P} = \ds\int_{t_i}^{t_f}\textbf{F}\,dt = \Delta\textbf{p}\) & (Impulse) \\
\( \ds\frac{dA}{dt} = \ds\frac{l}{2\mu}=cte \) & (Kepler's Second Law) \\
\end{tabular}
\end{document}

Answer 4

You can use the align environment.

Here is an example.

Edit: I have compiled this into a minimal working example, as per request. This will compile using XeLaTeX. I compiled this on a Mac running the latest MacTex distribution, but I think this should compile on most systems with XeLaTeX.

Edit^2: A bunch of includes are redundant, for instance, the enumerate include. I have copied the preamble from LaTeX-it.

\documentclass[10pt]{article}
\usepackage[usenames]{color} %used for font color
\usepackage{amssymb} %maths
\usepackage{amsmath} %maths
\usepackage[utf8]{inputenc} %useful to type directly diacritic characters
\usepackage{mathspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\setmainfont{Helvetica}
\setmathsfont(Digits,Latin,Greek){Helvetica}
\usepackage[margin=2in]{geometry}
\usepackage{enumerate}

\begin{document}
\begin{align}
\mathbf{P} = \int_{t_i}^{t_f}\mathbf{F}\,dt = \Delta \mathbf{p}\tag*{(Impulse) }
\end{align}

\end{document}

The align environment also gives you more fine grained control over the display of equations.