Hello from MrBillDoesMath!
Answer:
The first choice, Pi + (-Pi) = 0
Discussion:
Pi is an irrational number. Saying the irrational numbers are closed under addition means if we add two irrational numbers we will get another irrational number. In our case, Pi + (-Pi) = 0, which is correct, BUT 0 is a rational number. This shows the set of irrational numbers is NOT closed under addition.
Thank you,
MrB
