Explanation:
a, b are rational numbers.
c is an irrational number.
irrational numbers are all the real numbers that are not rational numbers. like square roots of not-squared numbers (e.g. sqrt(2), sqrt(3), pi, ...).
a + b is always rational.
a - b is always rational.
c² is sometimes rational (e.g. (sqrt(2))² = 2, but pi² is still irrational).
a/b is always rational for b <> 0.
(a)(b) is always rational.
ac is never rational.