Split an equation into two lines

Question

I have four consecutive equations. The third equation needs to be split into two. I need all the equations aligned. Also, instead of having each equation numbered separately, I want only one number indexing all the equations. I have written the following codes to implement it. However, the third equation's split does not look too nice. In addition, the right-hand side of each equation is very close to the "=" sign. Here is my output:

\begin{equation}
\begin{aligned}
SRI = & P\hat{P_x}+(1-P)(1-\hat{P_x}) \\
Var(SR)= & m^{-1}SRI(1-SRI) \\
Var(SRI) = & m^{-1}(2P-1)^2\hat{P_x}(1-\hat{P_x})+ m^{-1}(2\hat{P_x}-1)^2P_y(1-P_y)+ \\ 
& 4m^{-2}\hat{P_y}\hat{P_x}(1-P_y(1-\hat{P_x}) \\
P= & m^{-1}\sum_{j=1}^mI(\bar{\sigma}_{t+j})
\end{aligned}
\end{equation}

Could you suggest some improvements, please?

Answer 1

I would do this with alignedat and \widehat. Also, maybe I'm wrong, but I doubt that SRI and the like denote the product of the three variables S, R, I or Var is the product of V, a, r.

\documentclass{article}
\usepackage{mathtools}
\DeclareMathOperator{\Var}{Var}
\newcommand{\SR}{\mathit{SR}}
\newcommand{\SRI}{\mathit{SRI}}

\begin{document}

\begin{equation}
  \begin{alignedat}{2}
    \SRI &= P\widehat{P}_x+(1-P)(1-\widehat{P}_x) \\
    \Var(\SR) &= m^{-1}\SRI(1-\SRI) \\
    \Var(\SRI) &=
    m^{-1}(2P-1)^2\widehat{P}_x(1-\widehat{P}_x) & &{}+ m^{-1}(2\widehat{P}_x-1)^2P_y(1-P_y) \\
    & & &{}+ 4m^{-2}\widehat{P}_y\widehat{P}_x(1-P_y(1-\widehat{P}_x) \\
    P &= m^{-1}\sum_{j=1}^mI(\bar{\sigma }_{t+j})
  \end{alignedat}
\end{equation}

\end{document} 

Answer 2

Some suggestions and observations:

• In the fourth line, change & to &\quad, to provide some indentation of the line.

• Change all instances of = & to & = to improve the spacing around the = symbols.

• Change \hat{P_y} to \widehat{P}_y. Ditto for \hat{P_x}.

• Optional: If Var denotes "variance", it's a good idea to treat is a math operator (in TeX jargon) and to typeset using an upright rather than a slanted font.

• Optional: Typeset SRI and SR as variable names rather than as the products of the symbols S, R, and I.

Aside: I think a closing parenthesis is missing at the end of line 4.

\documentclass{article}  
\usepackage{amsmat h} % for '\DeclareMathOperator' macro and 'split` env.
\DeclareMathOperator{\Var}{Var}
\newcommand\vn[1]{\mathit{#1}}  % "variable name"
\begin{document}

\begin{equation}
\begin{split}
\vn{SRI}      &= P\widehat{P}_x+(1-P)(1-\widehat{P}_x) \\
\Var(\vn{SR}) &= m^{-1}\vn{SRI}(1-\vn{SRI}) \\
\Var(\vn{SRI})&= m^{-1}(2P-1)^2\widehat{P}_x(1-\widehat{P}_x)
                 + m^{-1}(2\widehat{P}_x-1)^2P_y(1-P_y) \\ 
              &\quad + 4m^{-2}\widehat{P}_y\widehat{P}_x(1-P_y(1-\widehat{P}_x) \\
P             &= m^{-1} \sum_{j=1}^m I(\bar{\sigma}_{t+j})
\end{split}
\end{equation}

\end{document}

Answer 3

Wrongly placed alignment tabs: they should be placed before the equal symbols.

My try:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{aligned}
SRI &= P\hat{P_x}+(1-P)(1-\hat{P_x}) \\
Var(SR) &= m^{-1}SRI(1-SRI) \\
Var(SRI) &= 
    m^{-1}(2P-1)^2\hat{P_x}(1-\hat{P_x}) + m^{-1}(2\hat{P_x}-1)^2P_y(1-P_y) +\\
    &\quad 4m^{-2}\hat{P_y}\hat{P_x}(1-P_y(1-\hat{P_x})\\
P &= m^{-1}\sum_{j=1}^mI(\bar{\sigma}_{t+j})
\end{aligned}
\end{equation}
\end{document}

Answer 4

I would typeset this as:

\begin{equation}
\begin{aligned}
SRI & = P\hat{P_x}+(1-P)(1-\hat{P_x}) \\
Var(SR)& = m^{-1}SRI(1-SRI) \\
Var(SRI) & = m^{-1}(2P-1)^2\hat{P_x}(1-\hat{P_x})+ m^{-1}(2\hat{P_x}-1)^2P_y(1-P_y)+ \\ 
& \qquad +4m^{-2}\hat{P_y}\hat{P_x}(1-P_y(1-\hat{P_x}) \\
P& = m^{-1}\sum_{j=1}^mI(\bar{\sigma}_{t+j})
\end{aligned}
\end{equation}

Note that I've written & =and not = &. Furthermore, I used a \qquad at the sixth line.