Question:
Let $x$ be rational and $y$ be irrational. Is $x y$ necessarily irrational? Justify your answer by an example.
Solution:
No, $(x y)$ is necessarily an irrational only when $x \neq 0$.
Let $x$ be a non-zero rational and $y$ be an irrational. Then, we have to show that $x y$ be an irrational. If possible,
let $x y$ be a rational number. Since, quotient of two non-zero rational number is a rational number.
So, $(x y / x)$ is a rational number $=>y$ is a rational number.
But, this contradicts the fact that $y$ is an irrational number. Thus, our supposition is wrong. Hence, $x y$ is an
irrational number. But, when $x=0$, then $x y=0$, a rational number.