Equation of Straight Line in Plane/Point-Slope Form
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Theorem
Let $\LL$ be a straight line embedded in a cartesian plane, given in slope-intercept form as:
- $y = m x + c$
where $m$ is the slope of $\LL$.
Let $\LL$ pass through the point $\tuple {x_0, y_0}$.
Then $\LL$ can be expressed by the equation:
- $y - y_0 = m \paren {x - x_0}$
Parametric Form
Let $\LL$ be a straight line embedded in a cartesian plane, given in point-slope form as:
- $y - y_0 = \paren {x - x_0} \tan \psi$
where $\psi$ is the angle between $\LL$ and the $x$-axis.
Then $\LL$ can be expressed by the parametric equations:
- $\begin {cases} x = x_0 + t \cos \psi \\ y = y_0 + t \sin \psi \end {cases}$
Proof
As $\tuple {x_0, y_0}$ is on $\LL$, it follows that:
\(\ds y_0\) | \(=\) | \(\ds m x_0 + c\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds c\) | \(=\) | \(\ds m x_0 - y_0\) |
Substituting back into the equation for $\LL$:
\(\ds y\) | \(=\) | \(\ds m x + \paren {m x_0 - y_0}\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds y - y_0\) | \(=\) | \(\ds m \paren {x - x_0}\) |
$\blacksquare$
Also presented as
This equation can also be seen presented as:
- $y - y_0 = \paren {x - x_0} \tan \psi$
where $\psi$ is the angle that $\LL$ makes with the $x$-axis.
Sources
- 1933: D.M.Y. Sommerville: Analytical Conics (3rd ed.) ... (previous) ... (next): Chapter $\text {II}$. The Straight Line: $2$.
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): line: 2.
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): point-slope form
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): line: 2.
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): point-slope form
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): line (in two dimensions) Point-slope form
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): straight line (in the plane)