For any number, including fractions, the additive inverse of that number is what you add to it to equal zero. For instance, 1 + -1 equals zero, so -1 is the additive inverse of 1 (and 1 is the additive inverse of -1).
For a fraction, this definition remains the same. For instance, 1/3 + -1/3 equals zero. Therefore, 1/3 is the additive inverse of -1/3, and -1/3 is the additive inverse of 1/3. Basically, if the number is positive, then the additive inverse will be the negative version of the same number, and the opposite is also true: if the number is negative, the additive inverse will be the positive version of that same number.
