If 5 plus 6 i is a root of the polynomial function f of x , which of the following must also be a root of f of x ?

For a polynomial with real coefficients, any nonreal complex root must occur with its complex conjugate. The conjugate of 5 + 6i is 5 − 6i, so that must also be a root. The other choices are not the conjugate of the given root.

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