Even though a Taser is a weapon that is not meant to kill, the prongs can still hurt people. There are some times when people die after they are shot with a Taser. This is because some people get hurt differently by electricity.

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In many places, people cannot own a Taser. There are special rules to follow to own a Taser. This is because it is considered a weapon.

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In Canada, France, Israel and the United Kingdom only police officers can have a Taser, but in the United States of America and Poland, most people can own a Taser.

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$(\text{area}\; \triangle ABC)$. This verbose form is perfectly acceptable in situations where it's not worth burdening the audience with an obscure symbol for something you won't use very often anyway. (I used this approach in this answer I recently posted.)

$|\triangle ABC|$. This is my notation of choice, as it fairly-obviously extends $|\overline{AB}|$ (and even $|-3|$), so that "$|x|$" means "the measure of $x$", whatever $x$ might be (number, segment, region, etc.). MathWorld uses it in their "Area Principle" entry. (The entry also uses square-brackets to represent "signed" ratios of areas.)

Tasers shoot two small pieces of metal called prongs. These prongs are connected to long pieces of metal string. When the Taser is shot, the prongs hit the person and electricity moves along the metal string to shock the person.

A Taser is a weapon that is not meant to kill. Tasers use electricity to shock people so they fall on the ground. If a person is shot by a Taser, they feel like they are hurt and cannot move their arms and legs. Tasers are used by police officers to stop people who want to hurt them, and to stop people who are running away.

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$[ABC]$ or $[\triangle ABC]$. This notation is not uncommon. I also see it here a good deal. Art of Problem Solving mentions this as alternative notation.

$S_{ABC}$. I see this notation fairly often here on Math.SE, but I dislike how it relegates the most important part of the symbol to a subscript. I'm not entirely sure why the "$S$" is used; for "surface", perhaps? Art of Problem Solving's "Area" page suggests subscripted "$A$" or "$K$". (It even mentions that $K_{ABCDEF}$ could denote the area of a hexagon.)

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Ultimately, you can use whatever notation meets your expositional needs. Given the variety of options out there, you best serve your audience by explicitly defining your usage on first appearance: Here we see that $|\triangle ABC| = 25$, where "$|x|$" indicates the area of $x$.

$(ABC)$ or $(\triangle ABC)$. I've seen this, but only very rarely; that's perhaps because parentheses are already quite overloaded in math notation (and text!).

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