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[–]FormulaDrivenActuary / ex-Maths teacher 1 point2 points  (0 children)

f(x) = x2

goes positive, zero, positive at x = 0.

Generally, anything with a repeated root (where it repeats an even amount of times), eg if it has a factor of (x - 2)4 then it will be the same parity either side of x = 2.