the reason you use letters in algebraic expressions is because you don't know the number so a letter replaces it.
I think it is important because you need to know it when you get older.
Variable is any letter used in an algebraic expression, and can vary (change form) to be any number, and one variable means the same number in any single algebraic expression. Usually algebra is simplifying the expression or equation until you know what the variable is equal to.
Learning the expressions is the same thing as learning your multiplication tables - it helps you to understand algebra. And algebra allows you to understand things like logic, statistics, and all sorts of calculations where you don't know enough numbers to do a straight addition or multiplication or subtraction or division.
An algebraic geometer is a mathematician who specializes in algebraic geometry.
You need to find the common denominator in order to add or subtract them. You can only add or subtract "like things" and by finding a common denominator you make both rational expressions into things that can be added or subtracted.
Replace each variables in the algebraic expression by its [known] value and calculate the value (ie evaluate) of the algebraic expression.
omg dont know
umm I don't know
i don't know something