In math, a triangle is written as the triangle symbol followed by three letters that represent the points of the triangle. For example, typically a triangle is denoted in an expression like this.
In plain language, the expression can be read as the triangle formed by the three points , and . Visually, this would correspond with a triangle like this:
When referring to a circle in a diagram the symbol ⊙ (odot) is used followed by the variable associated with the center point of the circle. For example, ⊙P would mean the circle whose center is located at the point P.
A line is written as two letter with a line overtop, where the letters represent the end points of the line.