Answer: True
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Step-by-step explanation:
Picking any two negative numbers, then adding them, will result in some other negative number. This is why the set of negative numbers is closed under addition.
Examples:
-2 + (-3) = -5
-10 + (-14) = -24
-7 + (-9) = -16
This is because when you add two negatives, you're adding their positive counterparts. Going to the first example above, we would say 2+3 = 5 and then just make everything negative. There's no way to add two negative numbers, and get a positive result (and we can't get 0 either).
You can think of it in terms of money and debt. The -2 represents being 2 dollars in debt. Then adding on 3 more dollars of debt leads ultimately to 5 dollars overall in debt.