Directed angles

Directed Angles is a method to express angles that can be very useful in angle chasing problems where there are configuration issues.

Definition

Given any two non-parallel lines $l$ and $m$ , the directed angle $\measuredangle(l,m)$ is defined as the measure of the angle starting from $l$ and ending at $m$ , measured counterclockwise and modulo $180^{\circ}$ (or say it is modulo $\pi$ ). With this definition in place, we can define $\measuredangle AOB = \measuredangle(AO,BO)$ , where $AO$ and $BO$ are lines (rather than segments).

An equivalent statement for $\measuredangle AOB$ is that, $\measuredangle AOB$ is positive if the vertices $A$ , $B$ , $C$ appear in clockwise order, and negative otherwise, then we take the angles modulo $180^{\circ}$ (or modulo $\pi$ ).

Figure 1: The directed angle $\measuredangle(l,m)=50^{\circ}$ , while the directed angle $\measuredangle(m,l)=-50^{\circ}=130^{\circ}$

Figure 2: Here, $\measuredangle ABC=50^{\circ}$ and $\measuredangle CBA=-50^{\circ}=130^{\circ}$

Note that in some other places, regular $\angle$ notation is also used for directed angles. Some writers will also use $\equiv$ sign instead of a regular equal sign to indicate this modulo $180^{\circ}$ nature of a directed angle.

Warning

The notation introduced in this page for directed angles is still not very well known and standard. It is recommended by many educators that in a solution, it is needed to explicitly state the usage of directed angles.

Never take a half of a directed angle. Since directed angles are modulo $180^{\circ}$ , taking half of a directed angle may cause unexpected problems.

Do not use directed angles when the problem only works for a certain configuration.

Important Properties

Oblivion: $\measuredangle APA = 0$ .
Anti-Reflexivity: $\measuredangle ABC = -\measuredangle CBA$ .
Replacement: $\measuredangle PBA = \measuredangle PBC$ if and only if $A$ , $B$ , $C$ are collinear.
Right Angles: If $AP \perp BP$ , then $\measuredangle APB = \measuredangle BPA = 90^{\circ}$ .
Addition: $\measuredangle APB + \measuredangle BPC = \measuredangle APC$ .
Triangle Sum: $\measuredangle ABC + \measuredangle BCA + \measuredangle CAB = 0$ .
Isosceles Triangles: $AB = AC$ if and only if $\measuredangle ACB = \measuredangle CBA$ .
Inscribed Angle Theorem: If points $A$ , $B$ , $C$ is on a circle with center $P$ , then $\measuredangle APB = 2\measuredangle ACB$ .
Parallel Lines: If $AB \parallel CD$ , then $\measuredangle ABC + \measuredangle BCD = 0$ .
Cyclic Quadrilateral: Points $A$ , $B$ , $X$ , $Y$ lie on a circle if and only if $\measuredangle AXB = \measuredangle AYB$ .

Application

The slope of a line in a coordinate system can be given as the tangent of the directed angle between $x$ -axis and this line. (Remember the tangent function has a period $\pi$ , so we have our "modulo $\pi$ " part in tangent function)

Other than that, direct angles can be very useful when a geometric (usually angle chasing) problem have a lot of configuration issues. We can avoid solving the same problem twice (sometimes even multiple times) by applying direct angles.

Here are some examples with directed angles:

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Directed angles

Contents

Definition

Warning

Important Properties

Application

See Also