Prove that the product of a non-zero rational and irrational number is irrational.

Solution 1:

As I mention here frequently, this ubiquitous property is simply an instance of complementary view of the subgroup property, i.e.

THEOREM $\ $ A nonempty subset $\rm\:S\:$ of abelian group $\rm\:G\:$ comprises a subgroup $\rm\iff\ S\ + \ \bar S\ =\ \bar S\ $ where $\rm\: \bar S\:$ is the complement of $\rm\:S\:$ in $\rm\:G$

Instances of this are ubiquitous in concrete number systems, e.g.

Solution 2:

You can directly divide by $q$ assuming the fact that $q \neq 0$.

Suppose $qy$ is rational then, you have $qy = \frac{m}{n}$ for some $n \neq 0$. This says that $y = \frac{m}{nq}$ which says that $\text{y is rational}$ contradiction.

Solution 3:

A group theoretic proof: You know that if $G$ is a group and $H\neq G$ is one of its subgroups then $h \in H$ and $y \in G\setminus H$ implies that $hy \in G\setminus H$. Proof: suppose $hy \in H$. You know that $h^{-1} \in H$, and therefore $y=h^{-1}(hy) \in H$. Contradiction.

In our case, we have the group $(\Bbb{R}^*,\cdot)$ and its proper subgroup $(\Bbb{Q}^*,\cdot)$. By the arguments above $q \in \Bbb{Q}^*$ and $y \in \Bbb{R}\setminus \Bbb{Q}$ implies $qy \in \Bbb{R}\setminus \Bbb{Q}$.

Solution 4:

It's wrong. You wrote $\frac{x}{z}y = \frac{a}{b}$. That is correct. Then you said "Therefore $xy = a$. That is wrong.

You need to solve $\frac{x}{z}y = \frac{a}{b}$ for $y$. You get $y = \frac{a}{b} \cdot \frac{z}{x}$.

Solution 5:

Let's see how we can modify your argument to make it perfect.

First of all, a minor picky point. You wrote $$qy=\frac{a}{b} \qquad\text{where $a$ and $b$ are integers, with $b \ne 0$}$$

So far, fine. Then come your $x$ and $z$. For completeness, you should have said "Let $x$, $z$ be integers such that $q=\frac{x}{z}$. Note that neither $x$ nor $z$ is $0$." Basically, you did not say what connection $x/z$ had with $q$, though admittedly any reasonable person would know what you meant. By the way, I probably would have chosen the letters $c$ and $d$ instead of $x$ and $z$.

Now for the non-picky point. You reached $$\frac{x}{z}y=\frac{a}{b}$$ From that you should have concluded directly that $$y=\frac{za}{xb}$$ which ends things, since $za$ and $xb$ are integers.