Boxing multiple equation

Question

\begin{equation}
\boxed{\sigma_{ann} \propto \begin{cases}
\frac{\lambda_3^2 m_b^2}{m_H^4} & \text{$m_S\ll\frac{m_H}{2}$}\\ \\
\frac{\lambda_3^2 m_b^2}{m_H^2 \Gamma_H^2} &\text{$m_S=\frac{m_H}{2}$}\\ \\
\frac{\lambda_3^2 m_b^2}{m_H^2 \Gamma_H^2} &\text{$m_S=\frac{m_H}{2}$}
\end{cases}}
\end{equation}

This is giving following output-

But i want my output to look like this

where size of fractions inside curly bracket is larger.

Answer 1

Answer to the updated question

Regarding the frac part of the question:

• If you use the amsmath package then you can use \dfrac instead of \frac.
• This will make is larger.
• displaystyle - hence the d in \dfrac - is the technical term I think.

Answer to the original question

Normally you should show some effort of your own or at least provide a so called MWE. But you are new and probably don't know that.

What you ask for is called cases environment (boxed version) - see the following related questions/answers for example:

• Using "cases" inside "equation" gives error[with "amsmath" loaded]

• eqnarray and cases

• How to write cases with LaTeX?

• How to write conditional equations with one-sided curly brackets (with an answer with 91 upvotes currently)
• Large braces for specifying values of variables by condition

There is even a tag (What is a tag?) for cases:

• https://tex.stackexchange.com/questions/tagged/cases

Boxed equations are also easy to find - here are some related questions/answers:

• How to make a box around an equation in align-environment
• How can I box multiple aligned equations?
• Equations and Equation Numbers within a Box

Answer 2

To get the "displaystyle", i.e., large, fractional expressions to the right of the curly brace, either use \dfrac instead of \frac or employ the dcases environment (requires the mathtools package) instead of the "basic" cases environment. As the other comments and answers have already noted, used \boxed to enclose the equation in a rectangular frame.

By the way, the \text{$...$} wrappers are unnecessary; just write out the argument.

\documentclass{article}
\usepackage{mathtools} % for 'dcases' environment
\begin{document} 
\begin{equation}
\boxed{
\sigma_{\mathrm{ann}} \propto 
\begin{dcases}
\frac{\lambda_3^2 m_b^2}{m_H^4}           & m_S\ll\frac{m_H}{2}\\[1ex]
\frac{\lambda_3^2 m_b^2}{m_H^2\Gamma_H^2} & m_S=\frac{m_H}{2}\\[1ex]
\frac{\lambda_3^2 m_b^2}{m_H^2\Gamma_H^2} & m_S=\frac{m_H}{2}
\end{dcases}
}
\end{equation}
\end{document}