Some mathematical operations have properties that can make them easier to work with and can actually save you time.
Some properties (axioms) of addition
You should know the definition of each of the following properties of addition and how each can be used.

Closure is when all answers fall into the original set. If you add two even numbers, the answer is still an even number (2 + 4 = 6); therefore, the set of even numbers is closed under addition (has closure). If you add two odd numbers, the answer is not an odd number (3 + 5 = 8); therefore, the set of odd numbers is not closed under addition (no closure).

Commutative means that the order does not make any difference in the result.
Note: Commutative does not hold for subtraction.

Associative means that the grouping does not make any difference in the result.
The grouping has changed (parentheses moved), but the sides are still equal.
Note: Associative does not hold for subtraction.

The identity element for addition is 0. Any number added to 0 gives the original number.

The additive inverse is the opposite (negative) of the number. Any number plus its additive inverse equals 0 (the identity).
Some properties (axioms) of multiplication
You should know the definition of each of the following properties of multiplication and how each can be used.

Closure is when all answers fall into the original set. If you multiply two even numbers, the answer is still an even number (2 × 4 = 8); therefore, the set of even numbers is closed under multiplication (has closure). If you multiply two odd numbers, the answer is an odd number (3 × 5 = 15); therefore, the set of odd numbers is closed under multiplication (has closure).

Commutative means the order does not make any difference.
Note: Commutative does not hold for division.

Associative means that the grouping does not make any difference.
The grouping has changed (parentheses moved) but the sides are still equal.
Note: Associative does not hold for division.

The identity element for multiplication is 1. Any number multiplied by 1 gives the original number.

The multiplicative inverse is the reciprocal of the number. Any nonzero number multiplied by its reciprocal equals 1.
; therefore, 2 and are multiplicative inverses.
; therefore, a and are multiplicative inverses (provided a 0).
A property of two operations
The distributive property is the process of passing the number value outside of the parentheses, using multiplication, to the numbers being added or subtracted inside the parentheses. In order to apply the distributive property, it must be multiplication outside the parentheses and either addition or subtraction inside the parentheses.
Note: You cannot use the distributive property with only one operation.