You must be able to identify and explain the difference between these key words:
Equation: An equation looks like this, x+3=5, the difference between this and an expression is the equal sign (=)
Formula: A formula is a special type of equation; it shows the relationship between two variables. A formula looks like this, v=hwl, when v = volume, h = height, w = width and l = length. x+3=5 is not a formula, it has only one variable x.
Identity: An identity is a relation which is true. This means that whatever the number or value may be, the answer stays the same. For example, algebraically, this occurs if an equation is satisfied for all values of the involved variables e.g 3(x+4) = 3x+12
Expression: An expression shows a mathematical relationship whereby there is no solution.
For example, 6x+3 is an expression there is no equals sign so there is no solution.