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In page Algebraic expression:

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By convention, letters at the beginning of the alphabet (e.g. a , b , c {\displaystyle a,b,c} ) are typically used to represent constants, and those toward the end of the alphabet (e.g. x , y {\displaystyle x,y} and z {\displaystyle z} ) are used to represent variables.[1] They are usually written in italics.[2]