calculus help please

Consider the function f(x)=2x+9x^–1.?
For this function there are four important intervals: (-inf,–A], [A,B),(B,C), and [C,inf)

where A, and C are the critical numbers and the function is not defined at B.

Find A= ?????
and B= 0
and C =?????

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  1. i have this one already

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  2. In that case I will assume that you no longer need help.

    Since f(x) = 2x + 9/x
    f'(x) = 2 - 9/x^2
    The function has extreme values at
    x = 3/sqrt2 and = -3/sqrt2
    Those are the points A and C.

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