i want to give equation number to multi-line equations

Question

am trying to group the equations then give them 1 number as group

  \begin{align*}
     &  \sigma_w^2(t)=q_1(t)\sigma_1^2(t)+q_2(t)\sigma_2^2(t)\\
         & \text{where} \\
      & q_1(t)=\sum_{i=1}^{t}P(i)  \:\& \: q_1(t)=\sum_{i=t+1}^{I}P(i)
    \\
     & mu_1(t)= \sum_{i=1}^{t}\frac{iP(i)}{q_1(t)} \: \& \: \mu_2(t)= \sum_{i=t+1}^{I}\frac{iP(i)}{q_2(t)}
    \\
     & \sigma_1^2(t)=\sum_{i=1}^{t}[i-\mu_1(t)]^2 \frac{P(i)}{q_1(t)} \: \& \: \sum_{i=t+1}^{I}[i-\mu_1(t)]^2 \frac{P(i)}{q_2(t)}\\
    \label{EqOtsu}
    \end{align*}

Answer 1

Your readers would have a hard time in understanding what the unique number refers to; I suggest using subequations.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{subequations}\label{EqOtsu}
\begin{equation}
\sigma_w^2(t)=q_1(t)\sigma_1^2(t)+q_2(t)\sigma_2^2(t) \tag{\ref{EqOtsu}}
\end{equation}
where
\begin{gather}
q_1(t)=\sum_{i=1}^{t}P(i)
\quad&\quad
q_1(t)=\sum_{i=t+1}^{I}P(i)
\
\mu_1(t)=\sum_{i=1}^{t}\frac{iP(i)}{q_1(t)}
\quad&\quad
\mu_2(t)= \sum_{i=t+1}^{I}\frac{iP(i)}{q_2(t)}
\
\sigma_1^2(t)=\sum_{i=1}^{t}[i-\mu_1(t)]^2 \frac{P(i)}{q_1(t)}
\quad&\quad
\sigma_2^2(t)=\sum_{i=t+1}^{I}[i-\mu_1(t)]^2 \frac{P(i)}{q_2(t)}
\end{gather}
\end{subequations}
\end{document}

Answer 2

You can use the aligned enivoment instead of align. aligned can be used inside an equation.

Thus the multi line expression can be obtained with:

\begin{equation}
  \begin{aligned}
     &  \sigma_w^2(t)=q_1(t)\sigma_1^2(t)+q_2(t)\sigma_2^2(t)\\
         & \text{where} \\
      & q_1(t)=\sum_{i=1}^{t}P(i)  \:\& \: q_1(t)=\sum_{i=t+1}^{I}P(i)
    \\
     & mu_1(t)= \sum_{i=1}^{t}\frac{iP(i)}{q_1(t)} \: \& \: \mu_2(t)= \sum_{i=t+1}^{I}\frac{iP(i)}{q_2(t)}
    \\
     & \sigma_1^2(t)=\sum_{i=1}^{t}[i-\mu_1(t)]^2 \frac{P(i)}{q_1(t)} \: \& \: \sum_{i=t+1}^{I}[i-\mu_1(t)]^2 \frac{P(i)}{q_2(t)}\\
    \label{EqOtsu}
 \end{aligned}
\end{equation}