Equation of a Line

Standard

$\begin{eqnarray*} Ax + By + C &=& 0 \end{eqnarray*}$

where:

the slope of the line is $-\frac{A}{B}$
the y-intercept is $-\frac{C}{B}$

Given two points $P_{1}$ and $P_{2}$ on the line, we can write that:

$\begin{eqnarray*} A &=& y_{1}-y_{2} \\ B &=& x_{2}-x_{1} \\ C &=& x_{1}y_{2}-x_{2}y_{1} \end{eqnarray*}$

in order to determine $A$ , $B$ and $C$ .

Slope-Intercept

$\begin{tikzpicture} % grid \draw[help lines] (-2,-2) grid (2,2); % origin \draw[red, line width=.1mm] (-0.1,-0.1) -- (0.1,0.1) (0.1,-0.1) -- (-0.1,0.1); \coordinate[label={[red]above left:$O$}] (O) at (0,0); \coordinate[label={[red]above left:$A$}] (A) at (-2,0); \drawpoint{A}{.5mm}{black} \coordinate[label={[red]above left:$B$}] (B) at (2,1); \drawpoint{B}{.5mm}{black} \draw (A) -- (B); \coordinate (C) at (2,0); \coordinate (D) at (0,1); \draw[dotted,->] (O) -- (C) node[below] {$x$}; \draw[dotted,->] (O) -- (D) node[left] {$y$}; \markangle{A}{B}{C}{3mm}{3mm}{$\alpha$}{cyan}{north} \end{tikzpicture}$

$\begin{eqnarray*} y &=& {\color{cyan}m}x + b & \\ \end{eqnarray*}$

where ${\color{cyan}m}$ is the slope:

$\begin{eqnarray*} {\color{cyan}m} &=& \tan{{\color{blue}\alpha}} \end{eqnarray*}$

where ${\color{blue}\alpha}$ is the angle between the line and the $\overline{Oy}$ axis.

${\color{cyan}m}$ can also be expressed parametrically as:

$\begin{eqnarray*} {\color{cyan}m} &=& \frac{y_{2}-y_{1}}{x_{2}-x_{1}} \end{eqnarray*}$

where $x_{1}, x_{2}, y_{1}, y_{2}$ are the components of the points $A(x_{1}, y_{1})$ and $B(x_{2}, y_{2})$ of the line.

Point-Slope

Given two points $A(x_{1}, y_{1})$ and $B(x_{2}, y_{2})$ and a connecting line $\overline{AB}$ , the point-intercept form of the line is given by:

$\begin{eqnarray*} y - y_{1} &=& m(x - x_{1}) \end{eqnarray*}$

where $m$ is the slope of the line.

Parametric Form

In parametric form:

$\begin{eqnarray*} P &=& P_{0} + t(P_{1} - P_{0}) \end{eqnarray*}$

where:

$P$ is a generic point on the line.
$P_{0}$ is the starting point of the line segment.
$P_{1}$ is the end point of the line segment.
$t$ is a parameter with $t: \mathbb{R} \mapsto [0, 1]$ where by taking values and working out the equation yields points on the line.

Midpoint of a Segment

$\begin{tikzpicture} % grid \draw[help lines] (-2,-2) grid (2,2); % origin \draw[black, line width=.1mm] (-0.1,-0.1) -- (0.1,0.1) (0.1,-0.1) -- (-0.1,0.1); \coordinate[label={[black]below left:$O$}] (O) at (0,0); \coordinate[label={[green]above left:$A(x_{1}, y_{1}, z_{1})$}] (A) at (-2,0); \drawpoint{A}{.5mm}{black} \coordinate[label={[blue]above left:$B(x_{2}, y_{2}, z_{2})$}] (B) at (2,1); \drawpoint{B}{.5mm}{black} \draw (A) -- (B); \coordinate[label={[red]above left:$M$}] (M) at (0,0.5); \drawpoint{M}{.5mm}{red} \coordinate (C) at (2,0); \coordinate (D) at (0,2); \draw[dotted,->] (O) -- (C) node[below] {$x$}; \draw[dotted,->] (O) -- (D) node[left] {$y$}; %\markangle{A}{B}{C}{3mm}{3mm}{$\alpha$}{cyan}{north} \end{tikzpicture}$

Knowing the components of two points $A$ and $B$ , we can determine the midpoint $\color{red} M$ of the segment $\overline{AB}$ whose components are:

$\begin{eqnarray*} \color{red} x_{M} &=& \frac{{\color{green}x_{1}}+{\color{blue}x_{2}}}{2} \\ \color{red} y_{M} &=& \frac{{\color{green}y_{1}}+{\color{blue}y_{2}}}{2} \\ \color{red} z_{M} &=& \frac{{\color{green}z_{1}}+{\color{blue}z_{2}}}{2} \\ \end{eqnarray*}$