# Line Equation in Intercept Form

## LaTeX code for a TikZ figure

This is a first draft of a TikZ picture illustraing this classical formula to be used for math help channels.

Adding \caption{for the picture} without “Figure 1:” requires \usepackage{caption} and wrapping with \begin{figure}. It also possible to use the primitive TeX command \par, but it would be complicated to use that with standalone. In the previous post, the SVG picture from the LaTeX table in an article has too much useless whitespace around the table. I don’t bother to learn other packages, as I need time for other more important stuff.

As a work around, I used \node at (x,y) {node content}. Adding [draw] before {node content} gives a rectangular box enclosing the node.

The grid lines need to be drawn before the axes. I learnt the technique of double grid lines from the classic TikZ pour l’impatitent. The [-latex] style comes from the LaTeX plugin in VS Code. I used [pos=1.05] instead of [right] and [above] to unify code the two axes. The graph of the staight line is restricted by \clip {TikZ command for a geomteric shape}, which is restricted by \begin{scope} and \end{scope}.

\documentclass[tikz,border=2pt]{standalone}
\begin{document}
\begin{tikzpicture}
\coordinate (O) at (0,0);
\coordinate (L) at (-2.5,0);
\coordinate (R) at (2.5,0);
\coordinate (D) at (0,-2.5);
\coordinate (T) at (0,2.5);
\coordinate (A) at (1,0);
\coordinate (B) at (0,1.5);

\node at (0, 3.5) [draw] {
Equation of straight line in intercept form
};

\draw[help lines, step=0.1] (-2, -2) grid (2,2);
\draw[gray, line width=0.75, step=0.5] (-2, -2) grid (2,2);
\draw[-latex] (L) -- (R) node[pos=1.05] {$x$};
\draw[-latex] (D) -- (T) node[pos=1.05] {$y$};
\begin{scope}
\clip (-2.5,-2.5) rectangle (2.5,2.5);
\draw[thick] plot[domain=-3:3] (\x,{-1.5*\x/1 + 1.5});
\draw (A) node {$\times$} node[below left] {$a$};
\draw (B) node {$\times$} node[below left] {$b$};
\end{scope}
\node at (0,-4) [draw, align=center] {
$a = x\textrm{-intercept}$ \\
$b = y\textrm{-intercept}$ \\ [1ex]
$\displaystyle\frac{x}{a} + \frac{y}{b} = 1$
};
\end{tikzpicture}
\end{document}