# Equation of Horizontal Line

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## Theorem

Let $\mathcal L$ be a horizontal line embedded in the Cartesian plane $\mathcal C$.

Then the equation of $\mathcal L$ can be given by:

- $y = b$

where $\tuple {0, b}$ is the point at which $\mathcal L$ intersects the $y$-axis.

## Proof

From the Normal Form of Equation of Straight Line in Plane, a general straight line can be expressed in the form:

- $x \cos \alpha + y \sin \alpha = p$

where:

- $p$ is the length of a perpendicular $\mathcal P$ from $\mathcal L$ to the origin
- $\alpha$ is the angle made between $\mathcal P$ and the $x$-axis.

As $\mathcal L$ is horizontal, then by definition $\mathcal P$ is vertical.

By definition, the vertical line through the origin is the $y$-axis itself.

Thus:

- $\alpha$ is a right angle, that is $\alpha = \dfrac \pi 2 = 90 \degrees$
- $p = b$

Hence the equation of $\mathcal L$ becomes:

\(\displaystyle x \cos \dfrac \pi 2 + y \sin \dfrac \pi 2\) | \(=\) | \(\displaystyle b\) | |||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle x \times 0 + y \times 1\) | \(=\) | \(\displaystyle b\) | Sine of Right Angle, Cosine of Right Angle | |||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle y\) | \(=\) | \(\displaystyle b\) |

Hence the result.

$\blacksquare$