Avoid vertical space between text and subequations

Question

I am using subequations environment with align and with eqnarray. In both cases, a large vertical space appears between text and equations. I don't know how to fixed that space to be the same as in equation environment...

Example:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\begin{document}

Teniendo en cuenta que podemos relacionar el ángulo $\theta$ (figura tal) con las componentes del vector normal: This is tooo much space:

\begin{subequations}
    \begin{eqnarray}
        a &=& \sin \theta \\
        c &=& \cos \theta = \sqrt{1 - a^2}
    \end{eqnarray}
    \label{eqangulos}
\end{subequations}

Podemos escribir el parámetro del rayo como... I want something like in equation environment:

\begin{equation}
    p = \frac{\sin \theta}{C} \left(1 + \frac{u \, \sin \theta}{C} \right)^{-1}
    \label{eqparametro3}
\end{equation}

This is good...

\end{document}

Answer 1

I have two suggestions:

• Don't allow paragraph breaks between text and a displayed equation. Remember that an all-blank line creates a paragraph break. (Depending on the material at hand, creating a paragraph break after a displayed equation can be ok, though.)

• Don't use eqnarray environments. Instead, use align environments.

\documentclass{article}
\usepackage{amsmath}  % for 'align' and 'subequation' environments
\begin{document}

Teniendo en cuenta que podemos relacionar el ángulo $\theta$ (figura tal) con las componentes del vector normal: This is good now:
\begin{subequations} % no empty line before this line
    \begin{align}    % and no empty line before this line either
        a &= \sin \theta \\
        c &= \cos \theta = \sqrt{1 - a^2} 
    \end{align}
\end{subequations}

Podemos escribir el parámetro del rayo como~\dots\ I want something like in an equation environment:
\begin{equation} % again: no empty line before this line
p = \frac{\sin\theta}{C} \left(1 + \frac{u\sin\theta}{C} \right)^{\!-1}
\end{equation}
This is also good.
\end{document}