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newcommand and math environement [migrated]
newcommand{twopartdef}[3]
{
left{
    begin{array}{ll}
        #1 & mbox{if } #2 \
        #3 & mbox{otherwise}
    end{array}
right
}

f$(x)$ = twopartdef{$lbrace alpha rbrace}{$x >$ 10}{lbrace beta rbrace}

documentclass{article}
newcommand{twopartdef}[3]{%
  left{
    begin{array}{ll}
      #1 & mbox{if } #2 \
      #3 & mbox{otherwise}
    end{array}
  right.
}
begin{document}
[
  f(x) = twopartdef{{ alpha }}{x > 10}{{ beta }}
]
end{document}


documentclass{article}
usepackage{amsmath}% http://ctan.org/pkg/amsmath
newcommand{twopartdef}[3]{%
  begin{cases}
    #1 & text{if #2} \
    #3 & text{otherwise}
  end{cases}
}
begin{document}
[
  f(x) = twopartdef{{ alpha }}{$x > 10$}{{ beta }}
]
end{document}