[Tex/LaTex] Reduce spacing between subequations

Question

How can I reduce Spacing of the following subequations:

$
\begin{subequations}
\begin{equation}
x_{n+1}= x_n + v_n \Delta t + \frac{1}{2} a_n \Delta t^2
\end{equation}    
\begin{equation}
 v\left(t_n+\Delta t/2\right)= v_n + \frac{1}{2}a_n \Delta t
\end{equation}
\begin{equation}
a_{n+1}=\frac{1}{m} F(x_{x+1},t_{n+1})
\end{equation}
\begin{equation}
v_{n+1}=v(t_n+\Delta t/2) + \frac{1}{2}a_{n+1}\Delta t
\end{equation}
\end{subequations}
$

I do not know how to embed the code here correctly.

Answer 1

As @daleif said, do not use separate equation environments, but the multiline (not multline!) environment gather if all equations have to be centred. In addition,the fraction coefficient are a little too big here, in my opinion.. You can use \mfrac (medium-sized fractions, ~ 80 % of \displaystyle) from nccmath to reduce spacing further:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath, nccmath}
\usepackage[french]{babel}

\begin{document}

\begin{subequations}
  \begin{equation}
    x_{n+1}= x_n + v_n Δt + \frac{1}{2} a_n Δt²
  \end{equation}
  \begin{equation}
    v\left(t_n+Δt/2\right)= v_n + \frac{1}{2}a_n Δt
  \end{equation}
  \begin{equation}
    a_{n+1}=\frac{1}{m} F(x_{x+1},t_{n+1})
  \end{equation}
  \begin{equation}
    v_{n+1}=v(t_n+Δt/2) + \frac{1}{2}a_{n+1}Δt
  \end{equation}
\end{subequations}

\bigskip

\begin{subequations}
  \begin{gather}
    x_{n+1}= x_n + v_n Δt + \mfrac{1}{2} a_n Δt²\\
    v\left(t_n+Δt/2\right)= v_n + \mfrac{1}{2}a_n Δt\\
    a_{n+1}=\mfrac{1}{m} F(x_{x+1},t_{n+1})\\
    v_{n+1}=v(t_n+Δt/2) + \mfrac{1}{2}a_{n+1}Δt
  \end{gather}
\end{subequations}
\end{document}