A or B must be an irrational number proof

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Prove that if a and b are real numbers such that the product of ab is an irrational number, then either a or b must be an irrational number. How do I prove this?

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Suppose the contrary: if a and b are rational, then their product is also rational (basic algebra $\frac{p}{q} \frac{r}{s}=\frac{pr}{qs}$)