# How do I make this long equation look neat?

Anybody knows how to make this long equation look neat in LaTeX? Many thanks!

\documentclass[12pt]{article}

\begin{document}
$$t_p= \left\{ \begin{array}{ll} 1 & D=0 \\ 1-(Credit_{fed}^L+Credit_{provin}^L) & 0<D\leq 200 \\ 200*[1-(Credit_{fed}^L+Credit_{provin}^L)] +(D-200)*[1-(Credit_{fed}^H+Credit_{provin}^H)] & D>200 \end{array} \right.$$
\end{document}

• Welcome to Maths SX! Could you post a full compilable code, as a starting point? – Bernard Nov 29 '17 at 0:05
• Start by using “C” instead of “Credit”. – egreg Nov 29 '17 at 0:07
• You should edit your question and insert this code therein. B.t.w. what are your document class, paper format, font size, &c.? – Bernard Nov 29 '17 at 0:20
• \documentclass[12pt]{article} – user149250 Nov 29 '17 at 0:23

If you want the variable names Credit, fed and provin to be in italics, use \mathit{Credit}, etc. Or \mathrm{Credit} for Roman.

Since you have a complicated repeated expression, I suggest you define a function C(K) to simplify your definition of t_p. I use the cases environment from the amsmath package.

Here is the code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
Let $C(K)=\mathit{Credit}_{\mathit{fed}}^K+\mathit{Credit}_{\mathit{provin}}^K$. Then
$$t_p=\begin{cases} 1 & D=0 \\ 1-C(L) & 0<D\leq 200 \\ 200\big(1-C(L)\big)+(D-200)\big(1-C(H)\big) & D>200 \end{cases}$$
\end{document}

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Something like this?

Observe the use of "roman", i.e., upright, lettering for the variable names 'Credit', 'fed', and 'provin'.

\documentclass[12pt]{article}
\newcommand\vn[1]{\mathrm{#1}} % "variable name"
\usepackage{mathtools} % for "dcases" environment
\begin{document}

$$t_p= \begin{dcases} 1 & D=0 \\[1ex] 1-(\vn{Credit}_{\vn{fed}}^L+\vn{Credit}_{\vn{provin}}^L) & 0<D\leq 200 \\[1ex] \begin{array}[b]{@{}r@{}} 200\phantom{)}[1-(\vn{Credit}_{\vn{fed}}^L+\vn{Credit}_{\vn{provin}}^L)]\\[1ex] {}+(D-200)[1-(\vn{Credi‌​t}_{\vn{fed}}^H+\vn{Credit}_{\vn{pr‌ovin}}^H)] \end{array} & D>200 \end{dcases}$$
\end{document}


I propose this non-standard layout, with some shortcuts, based o, the empheq package and the alignedat environment:

\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{empheq}
\newcommand{\credit}{\mathrm{Credit}}
\newcommand{\fed}{\mathrm{fed}}
\newcommand{\provin}{\mathrm{provin}}

\begin{document}

\begin{empheq}[left = {t_p = \empheqlbrace}]{equation}
\begin{alignedat}{4}
& 1 &\hspace{1.5em}\text{if }&D=0, &\hspace{4em} &
1-\bigl(\mathrm{Credit}_\mathrm{fed}^L + \mathrm{Credit}_\mathrm{provin}^L\bigr) &\qquad\text{if } & 0 <D\leq 200, \\[1.5ex] %
& \mathrlap{\begin{aligned}[b] 200*\bigl[1&-\bigl(\credit_{\fed} ^L + \credit_{\provin}^L\bigr)\bigr] \\& + (D-200)*[1-\bigl(\credit_{\fed}^H + \credit_{\provin}^H\bigl)\bigr]\end{aligned}} & & & & & \quad\text{if } & D>200.
\end{alignedat}
\end{empheq}

\end{document}