\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{$m_S=\frac{m_H}{2}$}

This is giving following output- enter image description here

But i want my output to look like this

enter image description here where size of fractions inside curly bracket is larger.

  • 1
    you can do this with \begin{equation} \boxed{...} \end{equation} – touhami May 19 '16 at 20:08
  • 1
    @touhami Maybe you should add that \boxed is a command from the amsmath package afaik. – Dr. Manuel Kuehner May 19 '16 at 20:15

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.

enter image description here

\usepackage{mathtools} % for 'dcases' environment
\sigma_{\mathrm{ann}} \propto 
\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}

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:

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

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

  • So is this question a duplicate then? – Werner May 19 '16 at 20:21
  • @Werner Yes, I think so (2 duplicated I guess, cases and boxed equation). – Dr. Manuel Kuehner May 19 '16 at 20:26
  • If you have a duplicate, then vote-to-close. – Werner May 19 '16 at 20:34
  • @Werner Done... – Dr. Manuel Kuehner May 19 '16 at 20:37
  • I have edited the question. – solphy101 May 19 '16 at 20:46

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