Answer:
6. The set of non-positive integers includes 0 while the negative integers do not.
7. 3 x π does not give a rational number, but another irrational which is 3π.
8. Addition and Multiplication
9. 9-(8-7) which is 8 is not equal to (9-8)-7 which is -6.
10. Zero. Any number added to zero is equal to the number itself.
11. 1/x.
