Algebra is the branch of mathematics that uses letters and symbols to represent numbers. These letters are called variables, and they allow us to write general relationships instead of specific numbers. When we evaluate an algebraic expression or a formula, we are simply finding its value after substituting specific numbers for the variables.
For example, if have an expression of 3x + 5, when x= 5, 3x+ 5 = 3(5)+5)=20.
A formula is a special type of algebraic expression that shows a relationship between quantities using an equal sign. For example, if A is area, and l is length, then the formula for the area of a square is given by A = l². If the length, l , is 5 cm, then we replace l with 5, and the area, A = 5² = 25 cm².