The additive inverse of ANY number is a unique number which when added to the first number results in Zero (0).
Before introducing this to your child, be sure they fully understand the entire section of numbers first, especially adding of both positive and negative numbers.
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If they don't they will find this aspect of basic algebra difficult. And believe me, because in algebra literally every section builds on the previous one, once they find one area difficult it wont be long before they are throwing their hands up at algebra all together!