Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/ia/paper-4/2e/solution

Suppose in lowest terms. Then . Comparing prime exponents shows that every prime dividing also divides , unless . Hence is an integer. Therefore is either an integer or irrational.
Only must be irrational: if it were rational, subtracting rational would make rational. None of the others must be irrational:
For , let . If is rational, is a counterexample. If is irrational, take and ; then . Thus is the sole expression forced to be irrational.

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