The additive inverse of a number, say a, is the number that, when added to a, gives zero.
The additive inverse of a real number is usually the negation (the number changes sign) of the original number.
i.e. if the number is a positive number, the additive inverse will be the negative value of the same number and if the original number is a negative number, the additive inverse will be the positive value of the same number.
Therefore, the additive inverse of 1/2 is -1/2.