We can simply observe that.
- 0.777... is rational, because it is a number with infinite but repeating decimal part.
- 1/3 is rational, because it's the division between two integers
, so this is rational as well.
Since the product of two rational numbers is always rational, we have that
are all rationals, since they are the product of two rationals.
On the other hand, we have
and thus
which is irrational.