Mathematics Handbook/Subtraction

1. a - 0 = a → Any number minus zero leaves that same number
2. a - (a) = 0 → Any number minus itself leaves zero
3. a - (-a) = 2a → Any number minus its opposite (additive inverse) leaves double the number
4. (-a) - (a) = -2a → Any negative number minus its opposite (additive inverse) leaves double negative of that number