Answer: Associative Property of Addition
Explanation:
Hey there! I will give you my reasons on why it is this answer, if you have any questions feel free to ask me in the comments below.
Question: 9 + (8 + 1) = (9 + 8 ) + 1 What property is this?
The answer to your problem would be the Associative Property of Addition. This is because it does not change the grouping of the addends, and it also does not change the sum. Good luck!
~I hope I helped you! :)~