title: How to label each equation in align environment?
body: <p>I wonder how to label each equation in align environment? For example</p>
  
  <pre><code>\begin{align} \label{eq:lnnonspbb}
  \lambda_i + \mu_i = 0 \\
  \mu_i \xi_i = 0 \\
  \lambda_i [y_i( w^T x_i + b) - 1 + \xi_i] = 0
  \end{align} 
  </code></pre>
  
  <p>only label the first equation and only the first equation can be referred later.</p>