Addition

Real numbers can be positive, negative, or zero. Addition of two negative numbers always results in a negative number and is carried out in the same fashion that positive numbers are added, after which a negative sign is placed in front of the result, such as -4 + (-21) = -25. Adding a positive and a negative number is the equivalent of subtraction, and, while it also proceeds by counting, the sum does not correspond to counting the members in the union of two sets, but to counting the members not in the intersection of two sets.