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jmc

algebra intermediate

Problem

Suppose that all four of the numbers are roots of the same nonzero polynomial with rational coefficients. What is the smallest possible degree of the polynomial?
Solution
Because the polynomial has rational coefficients, the radical conjugate of each of the given roots must also be roots of the polynomial. However, and are each other's radical conjugates, so we only get more roots. (You might be tempted to think that and are also a pair of radical conjugates, but the radical conjugate of is while the radical conjugate of is Therefore, each one of the numbers and is actually the negation of the radical conjugate of the other one.) In total, the polynomial must have at least roots.

Furthermore, the polynomial has roots and and has rational coefficients. Hence, the smallest possible degree is
Final answer
6