# Reducing space between for some of the equations

I need to display a few equations as below The code I have written is :-

\documentclass{report}
\usepackage{amsmath}

\begin{document}
\begin{equation} \label{Equ. 3.9}
x_{i+1}=\cos(a_i) \cdot [x_i-y_i \cdot 2^{-i} \cdot d_i]
\tag{3.9}
\end{equation}

\begin{equation} \label{Equ. 3.10}
y_{i+1}=\cos(a_i) \cdot [y_i+x_i \cdot 2^{-i} \cdot d_i]
\tag{3.10}
\end{equation}

\begin{equation} \label{Equ. 3.11}
\cos(\alpha)=\dfrac{1}{\sqrt {1+{\tan(\alpha)}^2}}
\tag{3.11}
\end{equation}

\begin{equation} \label{Equ. 3.12}
K_i=\cos(arctan(2^{-i}))= \dfrac{1}{\sqrt {1+\tan(arctan(2^{-   i}))}}=\dfrac{1}{\sqrt{1+2^{-2i}}}
\tag{3.12}
\end{equation}

The product of $K_i$  represents the so-called K factor (Equ. \ref{equ: 3.13})

\begin{equation} \label{equ: 3.13}
K=\prod K_i=\prod_{i=0}^{n-1} \dfrac{1}{\sqrt{1+2^{-2i}}}
\tag{3.13}
\end{equation}

\end{document}


The result I am getting with the above code is as below I need to reduce the space between the equations so the result looks like the 1st screenshot. Please guide how I can achieve this?

The equation that caused the numbering problem is:

\begin{equation}
\label{equ:matrix}
V=
\begin{bmatrix}
x'\\
y'\\
\end{bmatrix}
\begin{bmatrix}
x \cdot \cos(a) - y \cdot \sin(a)\\
y \cdot \cos(a) + x \cdot \sin(a)\\
\end{bmatrix}
\tag{3.6}
\end{equation}


When i remove the tag command from this it throws an error

• The equations displayed are also aligned by =, so the align environment would be best. You can still label and tag each equation individually, – John Kormylo Sep 2 '15 at 19:05
• Please don't post a substantially new query as an addendum to an existing posting. For one, relatively few people may realize that you've modified an existing query to ask a follow-up question. Instead, please post a new query. – Mico Sep 4 '15 at 16:20
• @Mico Sure... Sorry about that.. I am on it – Shray Sharan Sep 4 '15 at 16:22

Use the gather environment and the \intertext command. Btw, you don't have to put the equation numbers by yourself. Also, I propose to load the cleveref package: in cross references, you won't even have to type ‘equ.’, since cleveref knows (most) counters, and adds the counter name before its value.

\documentclass{report}
\usepackage{mathtools}

\begin{document}

\setcounter{chapter}{3}\setcounter{equation}{8}
\begin{gather}
\label{Equ. 3.9}
x_{i+1}=\cos(a_i) \cdot [x_i-y_i \cdot 2^{-i} \cdot d_i] \\
\label{Equ. 3.10}
y_{i+1}=\cos(a_i) \cdot [y_i+x_i \cdot 2^{-i} \cdot d_i] \\
\label{Equ. 3.11}
\cos(\alpha)=\dfrac{1}{\sqrt {1+{\tan(\alpha)}^2}}\\
\label{Equ. 3.12}
K_i=\cos(\arctan(2^{-i}))= \dfrac{1}{\sqrt {1+\tan(\arctan(2^{- i}))}}=\dfrac{1}{\sqrt{1+2^{-2i}}}\\
\intertext{The product of $K_i$ represents the so-called K factor (equ. \eqref{equ: 3.13})}
\label{equ: 3.13}
K=\prod K_i=\prod_{i=0}^{n-1} \dfrac{1}{\sqrt{1+2^{-2i}}}
\end{gather}

\end{document} • I used the tag command in one of the equations in between and from their it has stopped taking care of the numbering... Can i post the code for that equation in this post for you check it? – Shray Sharan Sep 4 '15 at 15:27
• Please do, since it is linked to this question. – Bernard Sep 4 '15 at 15:29
• I added the code for that equation.. – Shray Sharan Sep 4 '15 at 15:32
• I've inserted your equation code amidst the other equations and the result is normal, except number 3.6 between 3.12 and 3.13 looks weird. Could you post a complete code showing the problem (perhaps you should initiate a new thread)? – Bernard Sep 4 '15 at 16:07

Especially since you're already loading the amsmath package, you should look into using that package's gather environment to typeset a collection of displayed, numbered equations. Use \intertext{...} to intersperse text between some of the equations.

Some side remarks: For the sake of good (math) typography, consider getting rid of all \cdot directives and getting rid of the 2 radicands for the square root ops. \documentclass{report}
\usepackage{amsmath}
\allowdisplaybreaks
\begin{document}
\begin{gather}
x_{i+1}=\cos(a_i) \cdot [x_i-y_i \cdot 2^{-i} \cdot d_i]
\tag{3.9} \label{Equ. 3.9}\\
y_{i+1}=\cos(a_i) \cdot [y_i+x_i \cdot 2^{-i} \cdot d_i]
\tag{3.10} \label{Equ. 3.10}\\
\cos(\alpha)=\dfrac{1}{\sqrt {1+{\tan(\alpha)}^2}}
\tag{3.11} \label{Equ. 3.11} \\
K_i=\cos(\arctan(2^{-i}))= \dfrac{1}{\sqrt {1+\tan(\arctan(2^{-   i}))}}=\dfrac{1}{\sqrt{1+2^{-2i}}}
\tag{3.12} \label{Equ. 3.12}\\
\intertext{The product of $K_i$  represents the so-called $K$ factor (Equ. \ref{equ: 3.13})}
K=\prod K_i=\prod_{i=0}^{n-1} \dfrac{1}{\sqrt{1+2^{-2i}}}
\tag{3.13} \label{equ: 3.13}
\end{gather}
\end{document}