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http://www.math.northwestern.edu/courses/placement/220_Self_Placement.pdf [Broken]

Question 7 here involves a function with the rule f(x) = 3x

Question 7 here involves a function with the rule f(x) = 3x

^{5}-5x^{3}. I computed the 2nd derivative as f''(x) = 60x^{3}-30x (Mathematica agrees.), giving inflection points for f at -1/sqrt(2), 0, 1/sqrt(2). But the answer given in the PDF is 4x(4x^{2}- 3), whence they conclude the inflection points are -sqrt(3)/2, 0, sqrt(3)/2. Is this a mistake, or have I overlooked something?
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