Left Parenthesis Symbol
The left parenthesis is used to mark the start of an expression, or in the case of a function, a list of expressions.
Format  Display  Data 

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Usage
Grouping  Notation
In mathematics parentheses are used to group expressions to be evaluated together and organize the order of operations. Everything within the parantheses is evaluated, then the rest of the calculation is carried out.
Multiplication Parentheses  Notation
Two expressions placed next to eachother and separated by parenthesis are assumed to be multiplied together.
Function  Concept
A function takes input and produces output. The idea is a useful way to abstract away complexity and, especially in the age of computers, is a practical tool to solve problems.